Identification of rat liver phosphatidylinositol synthase as a 21 kDa protein

Substantial purification of rat liver phosphatidylinositol (PtdIns) synthase has been achieved by a combination of Hecameg extraction, heat treatment, affinity chromatography and chromatography on PBE-94. The activity chromatographs as a single peak which has an apparent molecular mass between 150 and 200 kDa on Sepharose 4B. When analysed by SDS/PAGE, two major bands are seen. The enzyme activity is correlated with a protein band of 21 kDa. A second band, at 51 kDa, is eluted from a PBE-94 column slightly ahead of the activity. Manganese is an absolute requirement for stabilization of activity in the presence of detergent. The effect of manganese is optimal at 0.5 mM; magnesium at a concentration of 10 mM is only minimally effective. Substrate Kms are 1.3 mM and 9.5 microM for inositol and CDP-diacylglycerol respectively. The activity eluting from the PBE-94 column is purified 5000-fold over the post-mitochondrial supernatant.