Neutral-alkaline proteases and protein degradation in rat heart

The role of neutral and alkaline proteases in protein degradation in heart muscle cells was assessed by measuring rates of proteolysis in perfused rat hearts, heart muscle cells, and homogenates of hearts from control rats and rats that were treated with Compound 48 80, a mast cell degranulator. Endogenous protein degradation in perfused hearts or in suspensions of heart muscle cells, as assessed by release of phenylalanine or disappearance of activity of S-adenosylmethionine decarboxylase in the presence of cycloheximide, was unaffected by treatment of the rat with Compound 48 80. The rate of phenylalanine release in heart muscle cells was similar to the rate that was observed in perfused hearts. Homogenates of control hearts that were incubated in the presence of Triton X-100 released phenylalanine over a pH range from 5 to 8 at rates that were equal to or greater than the rate of protein degradation that was observed in perfused hearts. At pH 5, only 30% of phenylalanine release was assayable in the absence of detergent. Homogenates of hearts from 48 80-treated rats and heart muscle cells released phenylalanine at pH 5 at the same rate as control homogenates, but at pH 7.8 the rate was only 10% of the value of control homogenates. These studies indicated that a large fraction of the alkaline proteolytic activity was in non-muscle cells and that this activity contributed little to overall protein degradation. These findings emphasize the importance of identifying the cellular localization of proteases before attempting to assess their role in myocardial proteolysis.