Intestinal absorption of amino acid derivatives: importance of the free alpha-amino group.

The intestinal absorption of L-lysine-p-nitroanilide, L-alanine-p-nitroanilide, and glycine-p-nitroanilide was studied in the presence of competitive inhibitors in a perfused rat intestine. It ws observed that L-lysine-p-nitroanilide absorption was inhibited by L-lysine methyl ester and L-arginine-beta-naphthylamide but not by N alpha-acetyl-L-lysine methyl esters. L-Alanine-p-nitroanilide absorption was inhibited by L-alinine methyl ester but not by beta-alanine methyl ester. It was further observed that N alpha-benzoyl-L-arginine-p-nitroanilide and N alpha-succinyl-L-phenylalanine-p-nitroanilide were poorly absorbed. It was concluded that the peptidase in the brush border region that serves as the hydrolysis site requires a free alpha-amino group (an aminopeptidase), and that passive absorption of these compounds occurs only to a small extent.