Alkaline ribonuclease and phosphodiesterase activity in rat liver plasma membranes

Abstract

The ribonuclease and phosphodiesterase activities of rat liver plasma membranes, purified from the crude nuclear fraction by centrifugation in an A-XII zonal rotor and flotation, were examined and compared. The plasma membrane is responsible for between 65 and 90% of the phosphodiesterase activity of the cell and between 25 and 30% of the particulate ribonuclease activity measured at pH8.7 in the presence of 7.5mm-MgCl(2). Both enzymes were most active between pH8.5 and 8.9. Close to the pH optimum, both enzymes were more active in Tris buffer than in Bicine or glycine buffer. Both plasma-membrane phosphodiesterase and ribonuclease were strongly activated by Mg(2+), there being at least a 12-fold difference between the activity in the presence of Mg(2+) and of EDTA. There is, however, a difference in the response of the enzymes to Mg(2+) and EDTA in that the phosphodiesterase is fully activated by 1.0mm-MgCl(2) and fully inhibited by 1.0mm-EDTA, whereas the ribonuclease requires 7.5mm-MgCl(2) for full activation and 5mm-EDTA for full inhibition. Density-gradient centrifugation has indicated that on solubilization in Triton X-100 most of the ribonuclease activity is released into a small fragment of the same size as that containing the phosphodiesterase activity. The relationship between the two activities is discussed in view of these results.