Comparative Digestive Physiology

Importance

Reptile Nutrition: From Mouth to Vent
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ADDITIONAL MEDIA

Feeding and Nutrition of Reptiles

These suggested concentrations are not sufficient to prevent signs of vitamin D deficiency in green iguanas. Vitamin C synthesis has been reported in many reptile species. It has been suggested that ulcerative stomatitis seen in snakes and lizards may be associated with a vitamin C deficiency, although there is no supportive evidence.

In controlled studies with garter snakes Thamnophis sp fed supplemental vitamin C, tissue levels and body stores remained stable, although synthesis by the snakes was reduced. Although most reptiles excrete nitrogen primarily as uric acid, aquatic reptiles typically excrete excess nitrogen as urea or ammonia. The relative proportions of various nitrogenous wastes may depend on the amount and composition of feed, frequency of feeding, and state of hydration. The excessive precipitation of urate crystals in joints, kidneys, or other organs gout can be a common condition in some species of captive reptiles.

The etiology is not clear, but it is commonly thought that diets high in protein may predispose reptiles to gout. Impaired renal function and dehydration have also been suggested as possible causes. If poor-quality protein is fed unbalanced amino acids or when tissue is catabolized for energy, uric acid excretion increases. Although gout in some reptiles is associated with increased circulating levels, postprandial transient increases in circulating uric acid may be seen in some species and confound the diagnosis.

Assuring an adequate state of hydration in a susceptible animal may help prevent uric acid precipitation in joints and organs. Feeding diets low in protein to carnivorous reptiles is unwise, because they are adapted to feeding on high-protein prey.

Most vertebrates can either absorb vitamin D from the diet or synthesize it in the skin from 7-dehydrocholesterol using energy from ultraviolet UVB light of certain wavelengths — nm in a temperature-dependent reaction. Thus, vitamin D is required in the diet only when endogenous synthesis is inadequate, as develops when animals are not exposed to UV light of appropriate wavelengths. Many captive basking species appear susceptible to rickets or osteomalacia.

Bone fractures, soft-tissue mineralization, renal complications, and tetany can develop. Reptiles frequently show few premonitory signs, although lethargy, inappetence, and reluctance to move are commonly reported. Serum calcium concentrations may not be diagnostically useful. Although blood levels of vitamin D can be measured, normal values for most species are not known.

Supplementation with injectable calcium and vitamin D may provide some short-term relief. However, exposure to UV light, or lack of it, may be an important, yet often overlooked, factor in the differential diagnosis. Complicating the diagnosis may be soft-tissue mineralization, seen radiographically or at necropsy. In green iguanas, metastatic calcification may not result from vitamin D toxicity.

Iguanas with both fractured bones and extremely low or undetectable levels of circulating hydroxycholecalciferol also had calcified soft tissues. The etiology of the metastatic calcification is not understood and is contrary to conventional understanding of the signs of vitamin D deficiency and toxicity in domestic species. Dietary sources of vitamin D may not be sufficient to prevent rickets and osteomalacia. Because some lizards seek a warm spot to increase body temperature, placement of a warming bulb, usually incandescent, adjacent to a UVB bulb helps ensure adequate exposure to UVB light.

Exposure to unfiltered natural sunlight, depending on latitude, during warmer months and use of UVB bulbs during the rest of the year usually eliminate the risk of bone disease caused by insufficient absorption of calcium due to a vitamin D deficiency.

These are some parts of it: Large intestine and small intestine Into the intestines there are 4 different functions: Chemic digestion of all the food The absortion of those nutrients The liver throws billis the pancreas througths pancreatic juice Functions Frogs Crocodilles Snakes Turtles Thank you very much Snakes' digestion The oral cavity may enlarge to swallow large pray thanks to the lack of attachment of the mandible, Snakes do no chew their pray, they swallow it whole.

The saliba of the snakes have reach enzymes that help to break down the food. The saliba also contains poison or venom. They are cold blooded. They have pulmonary and skin breathing. Creating downloadable prezi, be patient. Delete comment or cancel. Cancel Reply 0 characters used from the allowed. Send link to edit together this prezi using Prezi Meeting learn more: Reset share links Resets both viewing and editing links coeditors shown below are not affected.

But, hummingbirds are unremarkable in regards to other enzyme activities such as maltase and aminopeptidase-N. Maltase activity appears to be strongly correlated with diet among bird species. Nectarivorous and omnivorous species have higher maltase activities compared to insectivorous species , and, in phylogenetically informed analyses, maltase activity was positively correlated with dietary level of starch or seeds Pancreatic amylase was also significantly correlated with dietary starch level in a phylogentically informed comparison among six passerine species that consume diets with differing amounts of starch Improvements in molecular information have allowed better characterization of the changes in particular genes and proteins responsible for differences in digestive capacity.

These advances have been especially marked in studies of changes in carbohydrases coincident with inclusion of starchy foods and milk products in the human diet. In the case of starchy foods, the focus has been on salivary amylase. The salivary amylase gene Amy1 is closely related to the pancreatic amylase gene Amy2 from which it originated by duplication 8. Its function may be to i augment pancreatic amylase activity salivary amylase persists in the stomach after swallowing , or initiate starch breakdown in the mouth and thus either ii speed glucose absorption or iii release sugars for tasting and thus help in the identification of nutritious starchy foods 8 , Among humans sampled by Perry et al.

The populations were geographically widely distributed and the interpopulation variation in copy number was related most strongly to diet and not geographic proximity. Furthermore, AMY1 copy number and salivary amylase protein levels in humans generally are at least three times higher than in chimpanzees and bonobos, whose diets are composed predominantly of fruit and leaves that contain much less starch than the diets of most human populations.

The picture that emerges is one of correlated evolution of diet and amylase coincident with the dietary shift early in hominin evolutionary history toward starch-rich plant underground storage organs such as bulbs, corms and tubers and later to grains. Single-nucleotide polymorphisms SNPs seem to explain differences among human populations in the capacity to digest lactose in milk.

Milk is produced only by mammals, and its primary carbohydrate is lactose in most species. Lactose is hydrolyzed by the membrane-bound intestinal enzyme lactase-phlorizin hydrolase or lactase, for simplicity , which is coded by the lactase gene LCT.

In most mammals lactase activity is high at birth and declines sharply around weaning. Ingestion of large amounts of lactose post-weaning normally results in escape of undigested lactose to the distal GI tract where it is fermented, leading to production of gases CO 2 , H 2 , and methane and sometimes osmotic diarrhea.

The majority of humans are lactose intolerant, but members of a small number of populations that have been associated historically with domestic ungulates cows, sheep, and goats are lactose tolerant. The allele that carries the T variant was subsequently found to correlate with many global populations with lactose tolerance, and a variety of functional studies have revealed some of the molecular steps by which the allele controls the expression of lactase in intestinal cells But, there was more to the story because some populations e.

Subsequently, other SNPs were identified that correlated with lactose tolerance, and analyses seem to indicate that convergent evolution of the phenotype occurred a number of times at different locations Based on genetic patterns and analysis of Neolithic human skeletons, it seems that the ancestral human condition is lactose intolerance, but in a number of locations i. Genetic variants of amylase have been described in some invertebrates such as molluscs , and several insect species 12 , , There are practically no selection experiments designed to test for adaptation of digestive enzymes.

Flour beetles Tribolium castaneum that were raised on a variety of diets, whose carbohydrate contents likely varied but were not measured, showed some significant variation in amylase activity along with significant differences in growth rates and survival Recent findings about intestinal alkaline phosphate IAP have provided new insights about the former function, and intestinal lysozyme and pancreatic ribonuclease are key components of the latter function.

Alkaline phosphatase is found broadly across vertebrate and invertebrate taxa and in many organs within mammals, including intestine It is a brush border enzyme that hydrolyzes monophosphate esters, but its physiological role in digestion has not been well understood. For example, IAP-deficient mice have no apparent digestion deficits For many years its natural substrate s were not known, but its presence was widely used in intestinal studies as a marker of the apical brush border and as a marker for crypt-villus differentiation In , Poelstra et al.

In subsequent studies, IAP-deficient knockout mice and zebrafish 19 have been found to be hypersensitive to LPS toxicity compared with wild-type animals.

Dephosphorylation of LPS appears to inhibit its binding to receptors that initiate upregulation of inflammation-related genes that lead to inflammation and increased bacterial transmucosal passage , Lysozyme is another antimicrobial enzyme found broadly across vertebrate and invertebrate taxa in many kinds of tissues including the vertebrate intestine.

In that tissue, lysozyme and other bactericidal proteins called defensins are secreted by Paneth cells located at the base of intestinal crypts Lysozyme hydrolyzes the bacterial cell walls and the defensins insert into membranes where they interact with one another to form pores that disrupt membrane function and lead to the death of the bacterial cell But, another fascinating aspect of lysozymes is that they have been recruited as digestive enzymes over evolutionary time in several vertebrate and invertebrate taxa including foregut fermenting mammals and birds , insects 64 , , , and arachnids Acari Digesting microbes requires first breaking the bacterial cell walls and then hydrolyzing and absorbing the contents of the bacterial cell.

Bacterial cell walls are made primarily of peptidoglycan, which is hydrolyzed by the enzyme lysozyme. Most animals that assimilate their gut microbes have a compartment of the gut to culture the microbes and another one to digest them.

In at least two mammalian lineages and one avian species, the latter can be a site of lysozyme secretion. Ruminants, colobine monkeys, and hoatzins have evolved independently a lysozyme that functions as a digestive enzyme [reviewed in reference ]. This digestive lysozyme has many characteristics that distinguish it from the bacteriostatic lysozyme that is expressed in tears, milk, the Paneth cells of the small intestine, and in the whites of bird eggs.

The digestive lysozyme is expressed in the acidic compartment of the foregut, has an acidic pH optimum, and is relatively resistant to breakdown by pepsin [reviewed by reference ]. Colobine and ruminant lysozymes converged in the amino acid sequences that confer these enzymes their unique pH optima and pepsin resistance. The digestive lysozyme of hoatzins has a different genetic origin from that found in colobine monkeys and ruminants. Most mammals and birds have a single gene copy that codes for lysozyme.

Ruminants, in contrast, have many copies In ruminants, large-scale production of digestive lysozyme entailed both gene duplication and changes in the molecular structure of the protein. The activity of lysozyme in the stomach of the foregut fermenters is over three orders of magnitude higher than that found in animals with no foregut fermentation. Ribonucleases, secreted by the exocrine pancreas into the lumen of the small intestine, digest the abundant RNAs of rapidly growing bacteria. Although there has not been a good phylogenetically informed analysis, available evidence suggests that the ribonuclease content of the pancreas is higher in foregut fermenters and in some cecal fermenters that practice coprophagy than in omnivores and noncoprophagous herbivores [reviewed in reference ].

In addition, in ruminants and colobine monkeys the gene for ribonuclease duplicated, and one of the copies became specialized for the efficient digestion of bacterial RNA in the small intestine 23 , In this regard, it is interesting that rabbits secrete lysozyme in the distal colon under a circadian schedule that follows tightly that of the production of cecotrophs, which are the special pellets excreted from the cecum Thus, the cecotrophs that reach the stomach contain large amounts of lysozyme and, presumably, of bacteria with partially hydrolyzed cell walls ready to be digested.

A curious feature of the colonic rabbit lysozyme is that its pH optimum is very different from that of other lysozymes expressed in rabbits. It is acidic rather than neutral This observation suggests that in rabbits one of the lysozymes has been coopted from its original antibacterial role into the role of a digestive enzyme. The assimilation of bacterial protein by herbivorous birds is perplexing because birds do not seem to have spatial separation of culturing and digestion of microbes.

Also, to our knowledge no one has yet measured the activity of lysozyme in the GI tract of birds. Much remains to be learned about the mechanisms that vertebrate hindgut fermenters use to take advantage of their GIT microbes.

The GI tract of healthy animals is colonized by resident populations of microorganisms. In some animals, the gut microbiota contributes directly to nutrition by the fermentative degradation of plant cell-wall polysaccharides. Recent advances in sequencing technologies are transforming our capacity to study the diversity and function of the gut microbiota, and we consider these general issues first.

The taxonomic composition of the microbiota in the animal GI tract varies with phylogenetic position and diet of the animal, and with location in the GI tract , , Recent research on the diversity of the microbiota in the GI tract has been dominated by molecular analyses of bacterial diversity in the feces of humans and model rodent species, based on the assumption that fecal diversity is representative of the microbial community in situ.

The bacterial complement in mammals is dominated by two phyla, the Bacteroidetes and Firmicutes, each of which is represented by tens-to-hundreds of taxa, as identified by 16S rRNA gene sequence data Among humans, the composition varies widely among individuals, and is influenced by age 87 , , diet , and medical condition , including history of orally administered antibiotic treatment , Fecal analyses of a range of mammals reveal diet as an important determinant of taxonomic composition and genetic capacity for metabolism , such that the microbiota of mammals cluster according to whether the host is a carnivore, omnivore or herbivore, largely independent of the phylogenetic position of the mammal Fig.

Interesting outliers in this dataset are the pandas which, although folivores, have a microbiota that clusters with carnivores. In humans and other mammals, all regions of the GI tract are colonized, including the highly acidic stomach, which bears a diverse community of bacteria and some fungi Variation in bacterial communities of mammals with diet, analyzed by principal components analysis.

The analysis was conducted on individuals of 60 species from 13 orders of mammals. The three herbivores circled are individuals of red and giant panda, which are members of the order Carnivora. The species richness of the microbiota in the GI tract of many invertebrate animals is apparently an order of magnitude lower than in mammals, commonly with just 10 to 20 taxa per individual 7 , 22 , , , , , Nevertheless, the global diversity of microorganisms associated with the GI tract of invertebrates is substantial with different dominant species, phyla or even kingdoms in different animal taxa.

For example, the bacteria in the GI tract of Drosophila fruit-flies with a natural diet of rotting fruit are dominated by Acetobacter and Lactobacillus species 98 , , while the related tephritid Med fly, Ceratitis capitata , feeding on unripe fruits is colonized principally by Enterobacteriaceae, including Klebsiella , Pantoea , and Enterobacter species Analysis of the gut microbiota in Drosophila has revealed considerable variation in the dominant bacterial taxon with developmental age, even under uniform rearing conditions Fig.

The incidence of eukaryotic microorganisms e. Composition of bacterial species at different life stages of Drosophila melanogaster. Microorganisms in the GI tract of many animals have a great diversity of glucohydrolases active against complex plant polysaccharides.

Resident bacteria in the GI tract of humans also have considerable capacity to utilize carbohydrates, including complex plant polysaccharides. The genome of one common human gut symbiont Bacteroides thetaiotaomicron contains a total of glycoside hydrolases and polysaccharide lyases A metagenome analysis of fecal samples from 18 human individuals revealed a very diverse array of bacterial genes active against carbohydrates, collectively accounting for 2.

The relationship between the degradative capabilities of the bacteria in the GI tract and diet is further vividly illustrated by the discovery of genes for porphyranases and agarases in the gut bacterium Bacteroides plebeius isolated from Japanese but not North American individuals These enzymes are active against the sulfated polysaccharides in Porphyra seaweeds that form a regular part of the typical Japanese, but not North American, diet.

Furthermore, there is phylogenetic evidence that the genes for these glucohydrolase activities have been transferred horizontally from marine bacteria associated with Porphyra to the gut bacteria of humans.

The GI tracts of animals, including herbivorous mammals and wood-feeding insects, are recognized as cellulose-rich environments that are currently being targeted in gene discovery projects for biofuels development and other industrial purposes Microbial breakdown of complex carbohydrates can be nutritionally significant to the animal host, where the gut habitat is oxygen deficient, such that the microbial metabolism is strictly fermentative, and not aerobic.

Specifically, the complex polysaccharides are hydrolyzed to simple sugars, and then subjected to bacterial fermentation, with the net release of fermentation waste products, typically SCFAs, including acetate, butyrate, and propionate These final products diffuse across the animal gut wall, and are used as substrates for aerobic respiration, gluconeogenesis, and lipogenesis in the animal. The suite of reactions responsible for the transformation of complex carbohydrates to SCFAs is mediated by consortia of multiple bacteria with complementary capabilities , with cross-feeding of intermediate metabolites among bacteria with different capabilities Fig.

For example, in the human colon, Bacteroides species degrade complex polysaccharides to sugars; the sugars are respired by Bifidobacterium and other anaerobic bacteria to lactate; and the lactate is fermented by bacteria such as Eubacterium hallii and Roseburia hominis , producing butyrate Fig.

Butyrate, which is a waste product of the microbial community metabolism, is the principal respiratory substrate used by the gut epithelial cells Fermentative degradation of complex carbohydrates by consortia of bacteria in the human colon.

A Functional groups of bacteria SRBs, sulfate-reducing bacteria. B Major bacterial taxa responsible for degradation of starch and fructan-carbohydrates. Multiple factors beyond the biochemical capabilities of the microbiota determine the nutritional significance of microbial fermentation for an animal. Of particular importance are: All vertebrates apparently lack the capacity to degrade cellulose and related complex polysaccharides of plant cell walls.

Consequently, the amount of breakdown in the vertebrate GI tract is dictated by the scale of microbial fermentation, which varies from trivial, for example, in pandas Ailurus fulgens , A. Some invertebrate animals have enzymes capable of degrading plant cell-wall components. The phylogenetic distribution of intrinsic cellulases is not fully understood, but genome analyses indicate that members of at least five phyla have cellulases of glucose hydrolase family 9: The relative importance of intrinsic and microbial cellulolysis has been investigated, especially in insects , revealing considerable variation.

The capacity of some insects to degrade plant cell-wall components is further illustrated by the identification of enzymes from eight enzyme families capable to degrading plant cell-wall polysaccharides in a recent sequence analysis of seven species of phytophagous beetles Turning to the relationship between diet and microbial fermentation, various studies suggest that the taxonomic composition and metabolic traits of the gut microbiota can be influenced by diet, potentially with effects on the digestive function of the GI tract.

Indications that the microbial changes can be very rapid come from an analysis of laboratory mice with GI tract colonized by the microbiota from human fecal samples. Remarkably, the composition of the microbiota and gene expression profile was altered within a single day of transferring the mice from a low-fat diet with high plant polysaccharide content to a high-fat, high-sugar diet Although the entire length of the GI tract is colonized by microorganisms in most animals, the highest microbial densities and abundance tend to be in postgastric regions, for example, the large intestine of mammals, hind gut of insects, and this is the usual site of microbial fermentation chambers.

From the perspective of the animal, the key benefit of a postgastric fermentation chamber is that the substrates available to the microorganisms are those that are intractable to digestive action in the gastric region.

This design minimizes the competition between animal and resident microorganisms for ingested nutrients that can be processed readily by the animal. Pregastric fermentation chambers have evolved rarely, and are apparently restricted principally to mammals, with five independent evolutionary origins [in the Artiodactyls in the ruminants, camels, and hippos , in the colobine monkeys, and the Macropodidae kangaroos ]; the remarkable S American bird, the hoatzin, also has a pregastric fermentation chamber , The relative merits of pre- and postgastric fermentation have been discussed extensively , The key disadvantage of pregastric fermentation for the animal is that ingested food is available for microbial metabolism before digestion by the animal.

This can result in reduced nutritional gain from high-quality foods. For example, an animal derives more energy from simple sugars by gastric digestion and assimilation than by microbial fermentation; and more nitrogen from protein by gastric processing than microbial metabolism.

The adaptive advantage of pregastric fermentation for very efficient breakdown of the plant polysaccharides is enhanced by rumination i. It has been argued that pregastric fermentation chambers may have evolved in relation to functions other than cellulose degradation, for example to facilitate microbial detoxification of allelochemicals in ingested plant foods, and only subsequently became important in digestion of plant material Some animals possess a substantial fermentative microbiota that produces SCFAs without a morphologically distinct fermentation chamber.

This is particularly evident among herbivorous fish, including various tropical perciforms In one detailed analysis of three temperate fish species feeding on seaweed, the rate of production of one SCFA, acetate, was similar to those in the guts of herbivorous reptiles and mammals, even though the fish lacked coherent fermentation chambers Further research is required to determine the mechanisms underlying fermentation in these fish, and the nutritional significance of the SCFAs produced.

Absorption refers to the transfer of compounds from the gut lumen across the gut wall to the body tissues, including the lymph or blood of vertebrates and hemolymph of arthropods. At the cellular level, organic compounds can be absorbed from the gut lumen by paracellular and transcellular routes. Paracellular transport refers to movement between cells of the gut epithelium, while the transcellular route involves transport across the apical cell membrane of gut epithelial cells, transit across the cell for some molecules with metabolic transformations in the cell , and then export at the basolateral membrane.

This section considers absorption of organic compounds, particularly products of digestion: With the exception of SCFAs, these products are absorbed principally distal to the gastric region of the alimentary tract, for example, small intestine of vertebrates and midgut of insects.

The absorptive cells are columnar epithelial cells called enterocytes. Exceptionally, SCFAs produced by the microbiota in the hindgut e. In this section, two aspects of nutrient absorption are addressed: Most organic compounds absorbed across animal guts are polar, and their transport is predominantly or exclusively carrier-mediated, that is, mediated by membrane-bound transporters and displaying the twin characteristics of saturation kinetics and competitive inhibition.

Two forms of carrier-mediated transport are recognized: Simple diffusion, that is, down the concentration gradient and involving neither a carrier nor cellular energy, is an additional mode of absorption that is especially important for small, nonpolar molecules.

Monosaccharides cross the apical and basolateral membranes of gut epithelial cells by carrier-mediated mechanisms. Fructose is transported principally via the facilitative transporter GLUT5 These transporters are expressed predominantly in the small intestine.

The expression of SGLT1 in the intestine is restricted to the apical membrane of enterocytes. Once in the cell, the glucose is widely accepted to be transported down its concentration gradient across the basolateral membrane into the circulation by GLUT2.

Under conditions of high luminal glucose content, however, GLUT2 in rodents is inserted into the apical membrane, where it mediates the high flux of glucose into the enterocyte Some data suggest that sugar-induced translocation of GLUT2 may not occur universally in mammals 18 , , and further research is required to establish the distribution of this effect with respect to phylogeny and diet.

The mechanism by which GLUT2 is inserted into the apical enterocyte membrane is understood in outline This process occurs very rapidly. In the mouse, the responsiveness of GLUT2 insertion to luminal sugars varies among sugars, being triggered much less efficiently by glucose and complex sugars than by fructose, sucrose, and a mixture of glucose and fructose ; mice fed on a high-fructose diet have been reported to bear GLUT2 permanently on the apical membrane of enterocytes Artificial sweeteners, such as sucralose, dramatically increase GLUT2 insertion and the resultant uptake of glucose, such that the sugar is absorbed efficiently from lower concentrations in the presence of the artificial sweetener than in its absence The implications of these rodent studies for human nutrition are not yet fully resolved.

Phylogenetic analysis assigns the mammalian GLUT2 to a clade that includes three further mammalian GLUTs GLUT1, 3, and 4 and invertebrate, but no nonmetazoan, GLUTs, suggesting that this group of transporters may have evolved in the basal metazoans or immediate ancestors of animals There is also evidence that SGLT1 and GLUT transporters contribute to intestinal glucose absorption in nonmammalian vertebrates, including fish 72 , The molecular basis of sugar uptake across the gut wall has not, however, been investigated widely in the invertebrates.

Among insects, glucose transport across the midgut of the hymenopteran parasite Aphidius ervi is mediated by a SGLT1-like transporter on the apical membrane, together with a GLUT2-like transporter on both the apical and basolateral membranes of the enterocytes; and a second passive transporter similar to GLUT-5 is implicated in fructose uptake This condition is not, however, universal among insects.

These included an abundantly expressed gene ApSt3 , a hexose uniporter with specificity for glucose and fructose in the distal midgut.

Aphids may not, however, be typical of insects because their diet of plant phloem sap is sugar rich, and a concentration gradient from gut lumen to epithelial cell and hemocoel is maintained by the excess sugar in the gut lumen The products of protein digestion taken up by enterocytes of the mammalian intestine are free amino acids, dipeptides, and tripeptides.

Free amino acids are taken up from the small intestine of mammals by multiple carriers with overlapping specificities, with the result that most individual amino acids are transported by more than one transporter.

By contrast, peptides are taken up by a single transporter with very low selectivity, as considered at the end of this section. The amino acid transporters are classified by their activity specificity and kinetics into multiple systems, and by sequence homology into solute carrier SLC families.

The principal transporters mediating amino acid transport in the human intestine are summarized in Table 3. Studies on human, rodent and rabbit suggest that the amino acid transporters in the mammalian small intestine can be assigned to four groups, mediating the transport of neutral, cationic, anionic, and imino acids, respectively Uptake across the apical membrane is mediated by: The rich classical literature on the kinetics of amino acid transport across the intestinal epithelium of various nonmammalian vertebrates and invertebrates is summarized by and , and there is increasing interest in analysis from a molecular perspective [e.

As in mammals, multiple transporters are expressed, with overlapping specificities for amino acids. Some are very specific, for example, NAT6 and NAT8 in the distal midgut of mosquito Anopheles gambiae transport just aromatic amino acids , Other SLC6 transporters have a very broad range. Notably, the neutral amino acid transporter in Drosophila DmNAT6 can mediate the transport of most amino acids apart from lysine, arginine, aspartate, and glutamate; and, remarkably, it can also take up D-isomers of several amino acids This capability can be linked to the abundance of D-amino acids in the cell walls of bacteria, which are an important component of the natural diet of Drosophila species.

DmNAT6 is an active transporter, capable of mediating uptake against the concentration gradient. Multiple transporters are involved with a range of specificities, including two neutral amino acid transporters in Manduca sexta KAAT1 and CAATCH1 , both members of the SL6 family 71 , with distinctive amino acid selectivities Amino acid transporters are also expressed in the apical membrane of the insect hindgut epithelium, where they mediate the uptake of amino acids in the primary urine produced in the Malpighian tubules.

Proline is also taken up, and is a major respiratory substrate of rectal cells It can transport thousands of di- and tripeptides with low affinity and high capacity, but neither free amino acids nor tetrapeptides This property is intelligible from the structural features of the binding pocket of the protein, which can accommodate compounds with oppositely charged head groups carboxyl and amino groups separated by a carbon backbone of 0. Neutral and most cationic peptides are cotransported with one proton, while anionic peptides require two protons Peptides taken up into the enterocyte are hydrolyzed by a diversity of cytoplasmic peptidases Fig.

The peptides are hydrolyzed by multiple cytosolic hydrolases, and the resultant amino acids are exported via the basolateral membrane by multiple transporters see Table 3. The efflux of unhydrolyzed peptides across the basolateral membrane is mediated by peptide transporters that have not been identified at molecular level.

The peptide transporter family to which the mammalian PEPT1 protein belongs is ancient, with the defining peptide transporter motif PTR motif evident in proteins of bacteria, fungi, plants, and animals Analysis of basal animal groups is required to establish the evolutionary origin s of gut-borne peptide transporter s in metazoans.

Of central importance is the relative importance of peptide and amino acid uptake in the protein nutrition of the animal. The significance of PEPT1 for the protein nutrition of other animals remains to be established. In vertebrates, the absorption of lipid hydrolysis products and sterols is dependent on their incorporation into micelles formed in the lumen of the small intestine. Micelles are 4 to 8 nm diameter aggregations of the hydrophobic lipid products with bile acids, which act as amphipathic detergents and mediate the passage of the lipid products across the aqueous boundary layer to the apical membrane of intestinal enterocytes.

A proportion of the micelle-associated molecules pass across the apical membrane by simple diffusion, according to the concentration and permeability coefficient of each compound, but carrier-mediated transport is also involved. The dominant lipids in most diets are triacylglycerols TAGs , accompanied by small amounts of various polar and nonpolar lipids, including phospholipids, sterols, and the fat-soluble vitamins A and E.

The products of lipid digestion include free FAs, glycerol, monoglycerides, and lysophospholipids. Following uptake by diffusion and via transporters, these products are transported to the endoplasmic reticulum, where they are used to synthesize diacylglycerols DAGs , TAGs, phospholipids, cholesterol esters, etc.

They are then packaged with lipoproteins to form chylomicrons, which are passed through the Golgi apparatus for exocytosis.

In mammals, the chylomicrons are delivered to the lymphatic vessels. The mechanism of chylomicron assembly is reviewed by reference Of particular note are the transporters mediating sterol flux across the apical membrane of enterocytes.

In mammals, a steep diffusion gradient across the apical membrane is generated by acyl-CoA: There is now overwhelming physiological and molecular evidence for carrier-mediated uptake and also efflux across the apical membrane Fig. The key transporter mediating cholesterol uptake is Niemann Pick C1-like 1 NPC1L1 protein, identified initially as the transporter sensitive to ezetimibe, a highly specific and potent inhibitor of intestinal cholesterol absorption 6 , , However, overexpression of NPC1L1 in nonenterocyte cells has not yielded cholesterol transport activity, suggesting that additional proteins may be required to reconstitute a fully functional cholesterol transporter.

ABC transporters generally have 12 transmembrane domains, but each of ABCG5 and ABCG8 has just six transmembrane domains; transport activity is mediated by the heterodimer, comprising a transmembrane protein complex Cholesterol molecules that are not esterified in the endoplasmic reticulum are eliminated from the enterocyte to the intestinal lumen and voided via the feces. Absorption of cholesterol in mammalian intestine. These esterified products are incorporated into apolipoprotein apo Bcontaining chylomicrons in a microsomal triglyceride transport protein-dependent manner.

After further processing, the chylomicrons are released from the basolateral membrane by exocytosis. Mammals feeding on fungal or plant material need to process the dominant sterols in these foods: These sterols have the tetracyclic ring structure and side chain at C17, as in cholesterol, but the side chain in phytosterols is alkylated at C e.

In healthy individuals, dietary phytosterols reduce serum cholesterol levels, probably through their more efficient incorporation than cholesterol into micelles, resulting in reduced cholesterol uptake ; this is why sitosterol is sold as a functional food.

A dietary supply of cholesterol is not required by mammals, which can synthesize sterols de novo. Among invertebrates, most research on lipid absorption has concerned insects. The products of insect lipid digestion are absorbed principally across the midgut epithelium, although absorption in the foregut, e.

Lipid absorption in insects differs from vertebrates in several important respects. Unlike chylomicrons, lipophorin is not synthesized in enterocytes; it is localized in the hemolymph blood , where it acts as a shuttle delivering lipids to the fat body and other organs.

Lipophorin has been implicated in the transport of hydrocarbons, carotenoids, sterols, and phosopholipids, as well as DAGs. The products of lipid digestion in the gut of the spider Polybetes phythagoricus are taken up by cells of the midgut diverticulum, where they are processed to TAGs and phospholipids and exported via two distinct carriers: This class of lipid-related molecules is distinctive from other lipids in two important respects.

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