Amino acid and peptide absorption from partial digests of proteins in isolated rat small intestine.

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Abstract
Absorption of each of 16 amino acids, free and peptide-bound, was measured in isolated rat small intestine perfused with 5 partial digests of proteins. At low concentrations net absorption of each amino acid was proportional to its luminal concentration and independent of the nature of the amino acid. A series of 1st-order multiple regressions described the characteristics of absorption. Rate constants for disappearance of free and peptide-bound amino acids from the lumen were closely similar. Substantial back-flux of amino acids derived from peptide hydrolysis occurred. Of the amino-N entering the serosal tissue fluid, 60-70% probably had left the lumen as free amino acids. Intact peptides crossed the mucosa during absorption from a soybean hydrolysate and in substantial quantities during absorption from one casein digest but not from another. With other hydrolysates there was no evidence for passage of peptides to the serosa. In several cases there was a serious discrepancy between the amount of amino-N absorbed from the lumen and the amount accounted for as peptide or free amino acid in the serosal secretion. The characteristics of absorption were similar (apart from the exceptions above) for all the digests studied except for soybean hydrolysate.