Role of polyphosphoinositide synthesizing enzyme in intracellar signal transduction system

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It has been well known that various hormones and autacoids enhance the inositol phospholipids metabolism, leading to various cellular responses. In this case, the degradation of phosphatidylinositol 4, 5, -bisphosphate(TPI) to diacylglycerol and inositol 1, 4, 5,-trisphosphate has been proposed to be a primary event. But, since TPI is present as a trace amount, it becomes to be depletion very rapidly. Therefore, very rapid compensatory resynthesis of TPI from phosphatidylinositol(PI) is necessary to continue the hormonal responses. This step was catalyzed by two kinases,PI kinase and phosphatidylinositol 4-phosphate(DPI) kinase.We tried to purify the enzymes and classify the characterization of them. PI kinase was purified from rat brain, by Affigel-Blue, DEAE-Biogel, Sephacryl 300, ATP-affinity and Mono Q column chromatography. Molecular weight was found to be 120,000 dalton and the specific activity increased 1250-fold, optimal pH was 7.0 and <Mg^(2+)> was required for the activity. The enzyme can only phosphorylate PI but not DPI or diacylglycerol.