Absorption of amino acids in different parts of the small intestine in growing pigs. III. Absorption of constituents of protein hydrolysates.

The study was performed on pigs weighing 50-75 kg with temporarily isolated loops of the proximal, middle and distal small intestine. Isolated segments were perfused with enzymic hydrolysates of known amino acids in free form and bound to peptides and proteins. Four hydrolysates were used from: casein I (CI), casein II (CII), soya bean protein (S) and gelatin (G). The absorption of nitrogen from all hydrolysates was least in the proximal small intestine (up to 7 g/m/24 h); there were no significant differences in nitrogen absorption between the other parts. The nitrogen absorption from the hydrolysates varied and was highest for CI and lowest for G. The rates of absorption of various amino acids were different, and were influenced by the proportions and composition of proteins, peptides, and free amino acids in the hydrolysates. However, the rates of absorption of methionine, arginine, leucine and phenylalanine were higher than those of lysine, threonine and histidine irrespective of the hydrolysate source. The peptide fraction of the hydrolysates disappeared from the lumen of the intestine more rapidly than did the amino acid fraction; this suggests that proteases and peptidases in the intestinal mucosa play an important role in the process of amino acid absorption.