Study on the localization of proteases of mitochondrial origin

A marked proteolytic activity against casein can be demonstrated in rat liver mitochondria. The proteases degrading casein appear distributed between a sedimentable fraction (Po) and a soluble extract (So). Part of the soluble fraction activity, which may be recovered in the mitochondrial intermembrane space, results from a contamination by lysosomal proteases and can be eliminated by previously washing the mitochondria with digitonin. The pre-exposure to digitonin causes an enhancement of the caseinolytic activity associated with the membrane fragments, proving that this activity is not due to lysosomal enzymes. When rats have been injected in vivo with the compound 48/80 which, by degranulating the mast cells prevents contamination of the mitochondrial preparations by mast cell proteases, the membrane fraction (Po) retains a caseinolytic activity of the order of 80 per cent of the control preparations. A similar value of activity is observed in the membranes of brain mitochondria, isolated by a method which removes the rare mast cells they may contain. This shows that the greater part of the caseinolytic activity associated with the rat liver membranes does not originate from mast cell granules. Liver mitochondria pre-exposed to digitonin to eliminate lysosomal contaminants, have been subfractionated into matrix, intermembrane space, inner and outer membrane. Each of the fractions exhibits a caseinolytic activity, but the largest part is localized in the inner compartments of mitochondria: the matrix and the inner membrane. The optimal pH and the sensitivity to inhibitors of the proteases in the different compartments indicate that we are dealing with distinct enzymes.