Selective Cleavage of Peptide Bonds by a Serine Protease from the Muscle Layer of Rat Small Intestine1

Abstract

The kinetic constants of a serine protease from the muscle layer of rat small intestine for three ester substrates were compared with those reported for bovine chymotrypsin A. The K_m values for acetyl-tyrosine ethyl ester and acetyl-phenylalanine ethyl ester were very similar to those of chymotrypsin A, but the catalytic activity per mol of serine protease was only one fiftieth as high as that of chymotrypsin A. The selectivity of action of the serine protease from the muscle layer of small intestine was examined using various polypeptide substrates, such as glucagon, oxidized insulin B chain, luteinizing hormone-releasing hormone, and neurotensin. Among the peptides bonds cleaved in these substrates, the most susceptible bonds were Tyr-Leu, Trp-Leu, Phe-Phe, Tyr-Ile, and Tyr-Gly, while Phe-Tyr and Pro-Arg-Arg-Pro were less susceptible. However, unlike the chymotrypsin group, when the amino acid on the carboxyl side of tyrosine, tryptophan or phenylalanine was serine, threonine or glutamic acid, these peptide bonds were not susceptible to the protease under the experimental conditions. These results suggest that the specificity of the serine protease from the muscle layer of small intestine is that of the chymotrypsin group, but differs from that of chymotrypsin A or C.