Candida albicans phosphatidylinositol synthase has common features with both Saccharomyces cerevisiae and mammalian phosphatidylinositol synthases

Phosphatidylinositol (PI) synthase (cytidine 5′-diphospho (CDP)-1,2-diacyl-sn-glycerol:myo-inositol 3phosphatidyltransferase, EC 2.7.8.11) was isolated from the microsomal cell fraction of Candida albicans. The Triton X-100 extracted enzyme was enriched 140-fold by affinity chromatography on CDP-diacylglycerol–Sepharose. The enzyme had a pH optimum at 9·5 in glycine/NaOH buffer. It had an absolute requirement for Mg²⁺ or Mn²⁺ and was inhibited by Ca²⁺ and Zn²⁺. Maximal activity was at 0·2–0·6 mm-CDP-diacylglycerol, higher concentrations inhibited the enzyme. With 2′-deoxy-CDP-diacylglycerol as the lipid substrate, optimal activity was at 0·7 mm. The K_m for myo-inositol was determined to be 0·55 mm. The optimal temperature for the PI synthase reaction was 55°C. The C. albicans PI synthase shows differences to the Saccharomyces cerevisiae enzyme, such as activation by bivalent cations, inhibition by nucleotides, temperature optimum and activation energy, but also to the human PI synthase in preference for the lipid substrates, inhibition by nucleoside monophosphates and stabilization by Mn²⁺ and phospholipids.