Solubilized mitochondria and lysosomal fractions were obtained from guinea-pig heart by differential centrifugation and selective membrane disruption. 2. Mitochondria incubated at 37 degrees C in the presence of lysosomal enzymes underwent proteolysis. The rate of protein degradation was inversely dependent on pH. 3. The use of proteinase inhibitors showed that at low pH the major enzyme involved in mitochondrial digestion was cathepsin D. 4. At neutral pH carboxyl proteinases were still active, but thiol proteinases accounted for most of the protein breakdown. 5. The role of lysosomal enzymes as mediators of mitochondrial damage in ischaemic myocardium is discussed.
