Effects of diabetes on cardiac lysosomes and protein degradation

Rates of proteolysis by hearts obtained from alloxan diabetic rats and perfused as working preparations with buffer simulating control sera were accelerated 30% above identically perfused control hearts. The total homogenate activities of cathepsin D and beta-N-acetylglucosaminidase, assayed in the presence of Triton X-100, decreased 15-35% in diabetic heart, but the activities of these lysosomal enzymes assayable in the absence of detergent were unchanged in the diabetic tissue. The effects of diabetes were examined further by centrifugation of particulate fractions from subtilisin-treated hearts of control and diabetic rats on polyvinylpyrrolidone-coated colloidal silica (Percoll) gradients. Two species of lysosomes were resolved on the basis of their densities. Both dense and buoyant lysosomes accumulated radioactivity when hearts were exposed to [14C]phenylalanine methyl ester. Dense lysosomes (1.06-1.09 g/ml) sedimented with mitochondria while buoyant lysosomes banded with Golgi and sarcolemmal particles (1.02-1.03 g/ml). When particulate fractions of hearts from diabetic animals were layered on the Percoll gradients, total activities of beta-N-acetylglucosaminidase and cathepsin D were decreased from control in buoyant lysosomes, but unchanged in dense lysosomes. These results demonstrated that the increase in proteolysis in the diabetic heart was associated with decreased total activity and latency of cathepsin D and beta-N-acetylglucosaminidase and an increased proportion of dense lysosomes in the particulate fraction.