Effect of amino acids on purified rat intestinal brush-border membrane aminooligopeptidase

When a hexapeptide, Leu-Trp-Met-Arg-Phe-Ala, or a pentoapeptide, Leu-Trp-Met-Arg-Phe, was incubated in vitro with a purified aminooligopeptidase from rat small intestinal mucosa, the respective C-terminal dipeptides, Phe-Ala and Arg-Phe, were observed to be resistant to hydrolysis. The resistance of these C-terminal dipeptides to hydrolysis was found to be due mainly to the accumulation of inhibitory hydrophobic amino acids liberated in the incubation mixture. The hydrolysis of various peptides by the brush-border membrane peptidase is inhibited to a varying extent by the hydrophobic amino acids L-tryptophan, L-methionine, L-isoleucine, L-leucine, L-tyrosine, and L-phenylalanine, but not the D-form of these amino acids. The inhibition of the hydrolysis of three dipeptides by hydrophobic amino acids showed these amino acids to be competitive inhibitors (same Vmax, the maximal velocity of the enzyme reaction; different Km, the substrate concentration at which the enzyme reaction is half maximal) of one of the dipeptides while exhibiting a mode of inhibition that was not competitive (different Vmax, different Km) with either of the other two dipeptides. These data indicate that the effect of amino acids on the hydrolytic rate of the brush-border membrane aminooligopeptidases must be considered in studies of intestinal hydrolysis and absorption of peptides.