Hepatic Nucleases. 2. Association of Polyadenylase, Alkaline Ribonuclease and Deoxyribonuclease with Rat-Liver Mitochondria

Six types of nuclease activities were found to be concentrated in the large granule fraction isolated from rat liver homogenastes by differential centrifugation. Analysis by density equilibration shows that three nucleases are associated with mitochondria: an alkaline ribonulcease (pH optimum 8.8), an alkaline deoxyribonuclease (pH optimum 7.6) and an enzyme acting on polyriboadenylate (pH optimum 7.5). When the outer mitochondrial membrane is ruptured in hypotonic medium, the three mitochondrial nucleases are partially solubilized. Solubilization is however obtained by addition of KCL to the suspension medium. It is concluded that mitochondrial nucleases are localized in the intermembrane space but that an adsorption to the outer face of the inner mitochondrial membrane occurs in sucrose 0.25 M. The mitochondrial localization of alkaline ribonuclease, alkaline deoxyribonuclease and polyadenylate accounts for at least 80% of the activity of liver homogenate; nevertheless, an excess of these enzymes is present in the microsomal fraction. Although no definite conculusion can be reached for the significance of this observation, it is shown by density equilibration analysis that these nuclease are not associated either with ribosomes or with the membranes which are the major component of the microsomal fraction.