title: 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.
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology*; D002499:Centrifugation, Density Gradient; D002501:Centrifugation, Zonal; D004492:Edetic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000166:cytology / Q000187:drug effects / Q000201:enzymology*; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D051381:Rats; D012260:Ribonucleases / Q000378:metabolism*; D012995:Solubility; D013053:Spectrophotometry; D013056:Spectrophotometry, Ultraviolet
chemical_list: D004492:Edetic Acid; D012260:Ribonucleases; D010727:Phosphoric Diester Hydrolases; D008274:Magnesium