Features of lysosomal proteolytic enzyme activity in infarcted myocardium

Myocardial infarction was produced in dogs to investigate the behavior of cardiac lysosomal enzymes; acid protease and beta-glucuronidase activities were measured in whole myocardial homogenate and 30,000 X g supernatant fractions. Total activity of acid protease in the whole homogenate of the myocardium increased after 1 hr of infarction and showed maximal value after 6 hr, while in the 30,000 X g supernatant fraction the activity began to increase after 3 hr and reached the maximal level by 24 hr. Total beta-glucuronidase activity in the whole homogenate decreased during the initial 3 hr after myocardial infarction and markedly increased after 12 hr. The activity in the 30,000 X g supernatant fraction showed corresponding changes. These findings that the increase in whole homogenate activity preceded that in the 30,000 X g supernatant fraction may support the hypothesis that lysosomal enzymes may be newly synthesized and then solubilized in the infarcted myocardium.