title: Ca2+-sensitive phosphatidylinositol 4-phosphate metabolism in a rat beta-cell tumour.
abstract: Subcellular fractions were isolated from a rat beta-cell tumour by centrifugation of homogenates on Percoll and Urografin density gradients. Fractions were incubated with [gamma-32P]ATP, and labelling of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was used to measure phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase activities, respectively. The distribution of enzyme markers in density gradients indicated that phosphatidylinositol kinase was located in both the plasma membrane and the secretory-granule membrane. Phosphatidylinositol 4-phosphate kinase activity was low in all fractions. Phosphatidylinositol kinase activity of secretory granules and plasma membranes was decreased to 10-20% of its initial value by raising the free [Ca2+] from 1 microM to 5 microM. The enzyme had a Km (apparent) for ATP of 110 microM (secretory granule) or 120 microM (plasma membrane) and a Ka for Mg2+ of 7 mM (secretory granule) or 6 mM (plasma membrane). Ca2+-sensitivity of phosphatidylinositol kinase in calmodulin-depleted secretory granules and plasma membranes was not affected by addition of exogenous calmodulin, although activity was stimulated by trifluoperazine in the presence of 0.1 microM or 40 microM-Ca2+. Trifluoperazine oxide had no effect on the enzyme activity of secretory granules. Plasma membranes had a phosphatidylinositol 4-phosphate phosphatase activity which was stimulated by raising the free [Ca2+] from 0.1 to 40 microM. The secretory granule showed no phosphatidylinositol 4-phosphate-degrading activity. These results suggest the presence in the tumour beta-cell of Ca2+-sensitive mechanisms responsible for the metabolism of polyphosphoinositides in the secretory granule and plasma membrane.
mesh_terms: D007516:Adenoma, Islet Cell / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002462:Cell Membrane / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D003594:Cytoplasmic Granules / Q000378:metabolism; D066298:In Vitro Techniques; D007340:Insulinoma / Q000378:metabolism*; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D010766:Phosphorylation; D051381:Rats; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism; D014268:Trifluoperazine / Q000494:pharmacology
chemical_list: D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D010743:Phospholipids; C037178:phosphatidylinositol 4-phosphate; D014268:Trifluoperazine; D010744:Phosphoric Monoester Hydrolases; C053153:phosphatidylinositol-3,4-bisphosphate 4-phosphatase; D002118:Calcium