Substrate specificity of an adenohypophyseal endopeptidase capable of hydrolyzing luteinizing hormone-releasing hormone: preferential cleavage of peptide bonds involving the carboxyl terminus of hydrophobic and basic amino acids

The substrate specificity of a peptidase from anterior pituitaries that is capable of hydrolyzing luteinizing hormone-releasing hormone (LH-RH; less than Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) at the Tyr5-Gly6 peptide bond has been investigated by using inhibitors and model substrates. While trypsin and chymotrypsin inhibitors from plants and animals are without any effect, many microbial protease inhibitors and synthetic peptides containing hydrophobic and basic amino acids inhibit the degradation of radiolabeled LH-RH by this enzyme. The model substrates N-acetyl-Phe-Gly-Leu-beta-naphthylamide, N-acetyl-Leu-Gly-Leu-beta-naphthylamide, and N alpha-benzoyl-Arg-Gly-Leu-beta-naphthylamide are hydrolyzed at the X-Gly peptide bonds; N-acetyl-Gly-Gly-Leu-beta-naphthylamide is not degraded. Hydrolysis of typical amino- and carboxypeptidase substrates was not observed. Degradation of the general protease substrates insulin B chain and denatured hemoglobin also could not be detected. Thus, the enzyme is not LH-RH specific but may be characterized as an endopeptidase that hydrolyzes peptides preferentially at the carboxyl terminus of hydrophobic and basic amino acids.