Acid ribonucleases of lysosomal and soluble fractions from rat liver.

1. Lysosomal acid RNase was partially purified from a rat liver homogenate and a lysosomal fraction. This RNase, of optimum pH 5 to 6, cleaved RNA to form nucleoside 2’,3’cyclic phosphates, which were hydrolyzed to nucleoside 3’-monophosphates. This enzyme was heat-labile and had a molecular weight of about 24,000 to 28,000, as estimated by gel filtration with Bio-Gel P-60. This lysosomal acid RNase activity was not found in the supernatant of normal rat liver. 2. The location of this lysosomal RNase in the isolated subcellular fractions derived from normal liver was different from that found in fractions obtained from precancerous liver induced by 4-dimethylaminoazobenzene. In the latter, the activity was proved to be present in the supernatant fraction, as well as in the lysosomal fraction, as indicated by column chromatography and other enzymological studies. 3. Another acid RNase may be present in the supernatant fraction, although it could not be separated from the alkaline RNase of this fraction. This soluble acid RNase differed from lysosomal acid RNase in molecular weight and in the effects of various compounds on its activity. 4. The effects of ions and other compounds on the two acid RNases and rat liver alkaline RNase were compared. Lysosomal acid RNase was strongly inhibited by ZnClz, CuC12, and HgCIZ, whereas the other RNases were inhibited only slightly by these ions. 5. Ribosomes from rat liver did not inhibit lysosomal acid RNase, alkaline RNase from rat liver of Escherichia coli RNase I, although E. coli RNase I was inhibited by E. coli ribosomes.