THE DISTRIBUTION OF CALCIUM-DEPENDENT PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHODIESTERASE IN RAT BRAIN

Abstract— The properties of Ca2⁺-dependent phosphatidylinositol-phosphodiesterase in membrane fractions and supernatants prepared from rat brain have been examined with the aim of providing firm evidence for the existence of a membrane-bound activity distinct from the soluble enzyme found in the cytosol (EC 3.1.4.10). The soluble enzyme is either stimulated or inhibited at pH 7.0 by deoxycholate depending on the ratio of detergent to substrate. The effects of deoxycholate are pH dependent and result in a shift of the enzyme optimum to a higher pH if the enzyme is assayed in the presence of deoxycholate. The soluble enzyme cannot hydrolgse membrane-bound phosphatidylinositol (in ₃₂P-labelled rat liver microsomes) unless deoxycholate is present. The pH optimum is 6.7 for this detergent-dependent hydrolysis and this is probably dependent on the ionization of deoxycholic acid. The lactate dehydrogenase (EC 1.1.1.27) content of rat brain membrane fractions has been measured to estimate the contamination of these fractions by supernatant phosphatidylinositol-phosphodiesterase. No evidence has been found for phosphatidylinositol-phosphodiesterase activities that cannot be explained by such contamination. It is concluded that all the properties of calcium-dependent phospha-tidylinositol-phosphodicsterase in rat brain can be explained by the existence of only the solublc cyto-plasmic enzyme: no evidence confirming a distinct membrane-bound activity has been obtained.