Regulation of Cathepsin D Metabolism in Rabbit Heart: EVIDENCE FOR A ROLE FOR PRECURSOR PROCESSING IN THE CONTROL OF ENZYME ACTIVITY

Production of active lysosomal enzymes may involve limited proteolysis of inactive high molecular weight precursors. Precursor processing potentially regulates lysosomal enzyme activity. To test whether rabbit cardiac cathepsin D is first synthesized as a precursor and whether prolonged fasting (a condition affecting both cathepsin D and total cardiac protein turnover) influences precursor processing, rates of cathepsin D synthesis and processing were compared in left ventricular slices of control and 3-d-fasted rabbits incubated in vitro with [³⁵S]methionine. ³⁵S-labeled cathepsin D was isolated by butanol-Triton X-100 extraction, immunoprecipitation, and dodecyl sulfate-polyacrylamide gel electrophoresis. Total cardiac protein synthesis was measured by tracer incorporation and normalized for differences in precursor pool size by direct measurement of [³⁵S]aminoacyl-tRNA-specific radioactivity. Relative cathepsin D synthetic rates were obtained by comparing ³⁵S incorporation into cathepsin D with ³⁵S incorporation into all cardiac proteins. Enzyme processing was assessed in pulse-chase experiments and assayed by autoradiography. The results indicate that (a) rabbit cardiac cathepsin D is synthesized as a precursor (53,000 mol wt) that is processed to a 48,000-mol wt form, (b) rates of both cathepsin D and total cardiac protein synthesis are similar in control and fasted rabbits, suggesting that decreased enzyme degradation rather than increased synthesis is responsible for the elevated levels of cardiac cathepsin D in starvation, and (c) cathepsin D processing in hearts of fasted animals is incomplete, with accumulation of the precursor during pulse-chase experiments of 6 h duration. Based upon these results, a three-stage model for the regulation of cathepsin D activity in rabbit heart is proposed.