Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Nutrition, Digestion, and Absorption
Absorption of Amino Acids and Proteins

Under normal conditions, dietary Proteins are almost completely broken down into their constituent Amino Acids, which are then rapidly absorbed in the intestine. It is possible that certain hydrolytic processes (for example, in the case of dipeptides) are completed entirely within the intestinal wall. An animal's protein requirement can be successfully met by feeding a complete mixture of amino acids.

Natural (L-) isomers (but not D-isomers) of amino acids undergo active Transport Across the intestinal wall from the mucosal surface to the serosal side; vitamin B₆ (Pyridoxal phosphate) may be involved in this transport. The Active Transport of L-amino acids is an energy-dependent process, as evidenced by its inhibition by the Oxidative Phosphorylation uncoupler 2,4-dinitrophenol. Amino acids are carried across the brush border by a variety of transporters, many of which operate via Na⁺-dependent mechanisms, similar to the glucose transport system (Fig. 53.4). These Na⁺-dependent carriers include the neutral amino acid transporter, the phenylalanine and Methionine transporter, and a transporter specific for imino acids such as Proline and hydroxyproline. Na⁺-independent carriers specialized for The transport of neutral and lipophilic amino acids (e.g., phenylalanine and leucine) or cationic amino acids (e.g., Lysine) have also been characterized.

Clinical Aspects. Individuals who develop an immunological reaction to dietary protein apparently retain The ability to absorb a certain amount of unhydrolyzed protein, since digested protein lacks antigenic properties. This hypothesis is not entirely speculative, as it is well known that colostral Antibodies enter the bloodstream of an infant.

Increasing evidence Supports the hypothesis that in nontropical sprue, the primary defect is localized within the intestinal mucosal Cells and manifests itself in two ways: first, the Polypeptides formed during peptic and tryptic Digestion of gluten (the major wheat protein) exert a damaging effect on the intestine, and second, these polypeptides are absorbed into the bloodstream, which induces The formation of corresponding antibodies. Notably, antibodies against gluten or its fractions are frequently detected in the Blood of patients with nontropical sprue. The damaging effect is most likely attributable to a component consisting of a polypeptide of 6 or 7 amino acids, which must necessarily include glutamine and proline.

Class="center">Table 53.12. Sites of nutrient absorption

Analysis of this condition suggests that under certain circumstances, the intestine may absorb protein fragments of significantly larger molecular size than amino acids.

Tables 53.12 and 53.13 summarize data on the specific intestinal segments where various compounds are absorbed, and additionally provide information on the disorders resulting from malabsorption.