Mode of action towards oligopeptides and proteins of hydrolase H, a high-molecular-weight aminoendopeptidase from rabbit skeletal muscle.

The mode of action towards oligopeptides and proteins of hydrolase H purified from rabbit skeletal muscle was studied. The presence of protamine or alpha-N-benzoylarginine p-nitroanilide (an endopeptidase substrate) changed both the Km and V values of the enzyme towards Leu-beta-naphthylamide (an aminopeptidase substrate). This indicates that the binding site for an endopeptidase substrate is different from that for an aminopeptidase substrate. Hydrolase H as an aminopeptidase displayed broad specificity. The enzyme hydrolyzed various dipeptides readily except the dipeptides containing Pro or an amino acid with a hydrophobic beta-branched chain at the NH2 terminus. Pro and Val at the NH2 terminus of tripeptides were also difficult to release, whereas Ile and Val of tetrapeptides were easily released in contrast with those of dipeptides. The longer the peptide chain of Glyn (n = 2, 3, 4), the more susceptible was it to hydrolase H. Hydrolase H behaved as an endopeptidase only towards protamine among the proteins tested. The other proteins, casein, bovine serum albumin, myofibrils, troponin, hemoglobin, sarcoplasmic proteins, and myoglobin were probably attacked only by the aminopeptidase activity of the enzyme.