PROTEIN BREAKDOWN IN THE BRAIN. SUBCELLULAR DISTRIBUTION AND PROPERTIES OF NEUTRAL AND ACID PROTEINASES

Abstract

Optimum conditions for assay of rat-brain proteinases having pH optima of 3.8 and 7.6 were examined with respect to time of incubation and to substrate and enzyme concentrations. Comparison of proteinase activities of several tissue homogenates showed that the activity in brain was less than those in kidney, spleen or liver, but greater than that in muscle. On subcellular fractionation the highest activity of acid and neutral proteinases was in the mitochondrial fraction (P2), with lower activities in nuclei (P1), microsomes (P3), and ribosomal supernatant (S5). Sucrose-gradient centrifuging of the heterogeneous fraction P2 to decrease contamination revealed an association of acid proteinases with fractions sedimenting in 1-1.4 M-sucrose and known to contain true mitochondria, whereas neutral proteinases were associated with fractions containing myelin, nerve-ending structures and simple vesicles. Similar purification of P1 showed only a low activity associated with true nuclei. The subcellular distribution and the substrate specificity of proteinase suggest the presence of two separate enzyme systems. The possible presence of lysosomal particles in brain and the role proteinases may play in this organ are discussed.