Catalytic properties of a purified phosphatidylinositol-4-phosphate kinase from rat brain

Abstract

A phosphatidylinositol-4-phosphate (PIP) kinase activity was purified from rat brain extract through several chromatographic steps to yield an active preparation (specific activity 1 mumol of 32P incorporated into phosphatidylinositol 4,5-bisphosphate/min per mg of protein) with an apparent molecular size of 100-110 kDa in the native form. The isolated PIP kinase required Mg2+ (optimally 20-30 mM) for its activity and was not influenced by Ca2+. The enzyme used ATP (Km 25 microM) and GTP (Km 133 microM) as phosphate sources and appeared specific for PIP (Km 3.3 micrograms/ml) as the lipid substrate. The PIP-phosphorylation reaction was inhibited by micromolar concentrations of heparin [ID50 (concn. giving 50% inhibition) 2 micrograms/ml] and the flavonoid quercetin (ID50 0.2 microM). Whereas heparin behaves as a competitive inhibitor to PIP, quercetin was competitive towards ATP (or GTP). Phosphorylation of the preparation by a highly active purified protein kinase C did not detectably alter PIP kinase activity. Whereas 12-O-tetradecanoylphorbol acetate and various phospholipids had no effect, phosphatidylserine elicited a dose-dependent activation of PIP activity. This suggests that a phosphatidylserine-PIP kinase interaction may be considered as a possible regulatory process at the cell-membrane level.