Protein

Dietary protein intake varies greatly among individuals of like background and among groups with different geographic, cultural, and economic foundations.

Adult humans require 0.5-0.7 g of protein per kilogram of body weight per day to maintain nitrogen balance. Young children may require 4 g/kg per day.

In addition to dietary intake, protein is present in gastrointestinal secretions and the cells shed into the lumen of the digestive tract. This protein is handled in the same manner as dietary protein and accounts for an additional 35-55 g per day (10-30 g from secretions and 25 g from cells).

Essentially all ingested protein is assimilated. The capacity to digest and absorb protein greatly exceeds the dietary load, and normally dietary protein is absorbed completely before the end of the jejunum. The 10% of the total present in the tract that is excreted in the stool is primarily in the form of desquamated cells, bacteria, and mucoproteins, much of which is derived from distal regions of the tract.

Digestion

Two general classes of proteases are secreted into the lumen (Table 2). Endopeptidases hydrolyze interior

TABLE 2 Functions of Principal Luminal Proteases

Enzyme

Primary action

Endopeptidases

Pepsin Trypsin

Chymotrypsin

Elastase Exopeptidases

Carboxypeptidase A Carboxypeptidase B

Hydrolyze(s) interior peptide bonds containing Aromatic amino acids Basic amino acids; produces peptides with the C-terminal basic amino acids Aromatic amino acids, leucine, methionine, and glutamine; produces peptides with these amino acids at the C-terminus Neutral aliphatic amino acids; produces peptides with these at the C-terminus Hydrolyze(s) external peptide bonds containing Aromatic and neutral Aliphatic amino acids at the C-terminus Basic amino acids at the C-terminus peptide bonds, whereas the exopeptidases (both are carboxypeptidases) remove one amino acid at a time from the C-terminal ends of proteins and peptides.

Protein digestion begins in the stomach with the action of pepsin. The chief cells secrete the precursor pepsinogen, which is converted to pepsin by gastric acid (see Chapter 34). Three isozymes of pepsin have been identified, and each has a pH optimum between pH 1 and 3. All are irreversibly denatured after exposure to pH 5 or higher. Pepsin is inactivated when the chyme enters the duodenum and is neutralized by bicarbonate. Under optimal conditions, pepsin may result in the digestion of 10% of the protein present in the stomach. The contribution of pepsin to the digestion of a meal is dispensable, because patients with pernicious anemia, who secrete no acid, assimilate normal amounts of ingested protein. The pancreatic proteases are able to carry out all luminal digestion of protein.

The five major proteases of the pancreas are secreted as inactive precursors. Trypsinogen is converted to active trypsin by the brush border enzyme enterokinase (Fig. 6). Enterokinase cleaves a hexapeptide from the N-terminal end of trypsinogen to produce trypsin, an enzyme containing 223 amino acids. Trypsin is autocatalytic, and once enterokinase initiates the process, most of the remaining trypsinogen is activated by trypsin. Trypsin also converts chymotrypsinogen, proelastase, and the procarboxypeptidases to their active counterparts (Fig. 6). The principal actions of the pancreatic proteases are shown in Table 2. Therefore, luminal digestion of protein results in free amino acids plus small peptides of different lengths.

The pancreatic proteases also digest each other, so these enzymes are rapidly inactivated within the lumen.

These secreted proteins are digested and absorbed in the same manner as dietary proteins.

Absorption

Free amino acids produced by luminal digestion are absorbed across the enterocyte apical membrane. However, unlike carbohydrate absorption (in which only monosaccharides are absorbed), di- and tripeptides are absorbed intact. Larger peptides are absorbed poorly or not at all, and these are reduced to amino acids and di- and tripeptides by peptidases located in the brush border membrane. The digestion products of the brush border hydrolases are rapidly absorbed. These processes are summarized in Fig. 7. The absorption of whole proteins and large peptides is so minor as to be insignificant nutritionally. Absorption of whole proteins, however, is significant immunologically, and in some species other than humans the neonatal intestine has a high capacity to absorb whole proteins and especially immune globulins from colostrum. Absorption takes place by a pinocytotic mechanism and decreases with age.

Although a small amount of amino acids is absorbed by passive diffusion, L-isomers of most amino acids are absorbed by carrier-mediated secondary active transport systems requiring Na + . This mechanism is analogous to that previously described for the absorption of glucose and galactose (see Fig. 5). Studies of competition between different amino acids have identified separate carrier systems requiring Na+ (Table 3). Note that there is some overlap in amino acid specificity

FIGURE 6 Means of activating the pancreatic proteases that are secreted as proenzymes. Trypsinogen is activated by enterokinase. The resulting trypsin converts the other proenzymes plus additional trypsinogen.

[ Protein J

[ Protein J

FIGURE 7 Summary of protein digestion and absorption. Approximately 40% of ingested amino acids are absorbed as free amino acids or di- and tripeptides after luminal digestion. The remaining 60% are absorbed after being broken down further by membrane-bound peptidases. Free amino acids leave the cell by facilitated diffusion and by simple diffusion.

FIGURE 7 Summary of protein digestion and absorption. Approximately 40% of ingested amino acids are absorbed as free amino acids or di- and tripeptides after luminal digestion. The remaining 60% are absorbed after being broken down further by membrane-bound peptidases. Free amino acids leave the cell by facilitated diffusion and by simple diffusion.

when presented as an equal amount of free amino acids. Furthermore, amino acids presented to the gut as di- or tripeptides do not compete for transport carriers with identical free amino acids. These data indicate that carriers different from those for amino acids mediate the uptake of small peptides. The different peptide systems have not been well defined, but at least some appear to be secondary active transport systems dependent on Na+. After a meal, most of the protein is absorbed in the form of di-and tripeptides.

Within the enterocytes, most di- and tripeptides are hydrolyzed to amino acids by cytoplasmic peptidases. Free amino acids diffuse across the basolateral membrane or cross it by carrier-mediated processes. A number of carriers exist in the basolateral membrane. Some of these require Na+ and others that are identical to those in the apical membrane do not. In general, the more hydrophobic an amino acid, the greater the proportion that leaves the cell by diffusion. A small percentage of peptides is not broken down and enters the extracellular space intact. This entire scheme for protein digestion and absorption is summarized in Fig. 7.

among these carriers, and that the energy for the active accumulation of the amino acids comes ultimately from the (Na + ,K+)-ATPase of the brush border membrane. Other amino acids and some of those transported actively can be absorbed by facilitative processes that do not require Na+.

Although the uptake of free amino acids is significant, the transport of equimolar amounts of amino acids in the form of di- or tripeptides is more rapid and more efficient. Amino acids appear more rapidly in the portal blood when fed as di- or tripeptides than as free amino acids. A higher proportion of amino acids is assimilated when presented to the gut as a protein hydrolysate than

Neutral Neutral, aromatic, aliphatic

PHE Phenylalanine, Methionine

Acidic Aspartate, glutamate

Imino Proline, hydroxyproline y + Basic

L Hydrophobic neutral

A Neutral

ASC 3 or 4 carbon neutral

L Hydrophobic neutral y + Basic

ASC, alanine serine cysteine plus other 3 or 4 carbon neutral acids.

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