Properties of lysosomes in guinea pig heart: subcellular distribution and in vitro stability.

Some 20 hydrolytic enzymes, known to be of lysosomal origin in rat liver, were assayed in fractions of guinea pig heart. The enzymes included glycosidases, esterases, proteases (cathepsins) and nucleases. All exhibited structure-linked latency and had optimal activity in acid pH, conforming to the biochemical definition of lysosomal enzymes. Six enzymes which had high activity in guinea pig heart were also measured in rat liver using the fractionation and assay methods employed for myocardium. The acid hydrolase activities of the two tissues are compared and discussed in relation to the cellular origins of the organelles with which they are associated.A lysosome-rich suspension was prepared from guinea pig left ventricle by differential centrifugation. The isolated lysosomes were subjected to various labilizing conditions and the rate of lysosome disruption was monitored by measurements of latent acid hydrolase activity. Loss of latent activity was accelerated by heating, hypotonicity, changes in pH and by addition of NaCl and KCl. The kinetics of lysosomal enzyme release were similar for each of the enzymes tested, with the exception of β-glucuronidase which was released more rapidly by heating and less rapidly by hypotonicity.Subcellular distribution profiles of the acid hydrolases in guinea pig heart were obtained by sucrose density gradient centrifugation. Hearts from both normal and Triton WR 1339 treated animals were fractionated. The modal equilibrium densities of the acid hydrolases revealed the presence of five biochemically distinct lysosomal populations in ventricle tissue, three of which were accessible to Triton WR 1339.The findings in this study show that myocardial lysosomes are a heterogeneous group of organelles with different enzymatic constituents and different physical properties. In view of this there is a need for careful choice of representative marker enzymes in studies on the functions of myocardial lysosomes and their role in disease.