Proteolytic activity of rat skeletal muscle. I. Evidence for the existence of an enzyme active optimally at pH 8.5 to 9.0.

A considerable number of intracellular animal proteases have been described (1,2). Many of the enzymes studied exhibit pH optima in the acid range, whereas some show optimal activity at or near neutrality. Relatively few of the cathepsins described to date show maximal activity in the alkaline pH range. One such protease, found in aqueous extracts of whole rat lung, exhibits a pH optimum at 8.4 for the hydrolysis of urea-denatured hemoglobin (3). Another enzyme, found in erythrocytes, attacks hemoglobin optimally at pH 8 (4). Smith (5) has reported that aqueous extracts of rat skeletal muscle show little or no proteolytic activity against the endogenous muscle proteins of the extract in a pH range of 4.0 to 8.0. In the studies reported in this paper, it will be shown that rat skeletal muscle homogenates do, in fact, contain a proteolytic enzyme which readily attacks the endogenous muscle proteins at an optimal pH of 8.5 to 9.0. A preliminary account of this work has already been published (6).