As already indicated, although the final end product of protein digestion is amino acids, small peptides are the dominant form of entry of amino acids into enterocytes, where they are further hydrolyzed into amino acids and absorbed into the bloodstream (Figure 4). Thus, the vast majority of products of protein digestion that reach the bloodstream are single amino acids. Amino acid transport systems develop in utero by the end of the first trimester, whereas peptide transport systems can be demonstrated by the beginning of the second trimester.
It is recognized that the intestinal permeability of the preterm and newborn infant may be high, allowing the entry of small amounts of undigested proteins. The maternal antibodies from colostrum can enter the newborn's bloodstream relatively unaltered by a process of endocytosis and subsequent exocy-tosis. Although the intestinal permeability decreases with age, adults can still absorb larger proteins in abnormal circumstances. However, the predominant form of absorption and presentation of large foreign proteins is through the specialized microfold or M cells overlying the lymphoid Peyer's patches. This mode of absorption of intact proteins or polypep-tides, however, is nutritionally insignificant.
Di- and tripeptides can cross the brush border membrane by a main peptide transport system with broad specificity. This carrier protein can transport dibasic as well as diacid peptides and peptides consisting of up to three amino acid residues. However,
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