Abstract
Addition of isoproterenol to isolated rat adipocytes prelabeled with [32P]phosphate caused an increase in the phosphorylation and activation of phospholipid methyltransferase. 32P-Labeled phospholipid methyltransferase was recovered by immunoprecipitation and gel electrophoresis. Analysis of 32P-labeled peptides revealed one site of phosphorylation regulated by isoproterenol, and analysis of phosphoamino acids demonstrated that the incorporation of [32P]phosphate was on phosphoserine. Incubation of adipocytes with isoproterenol in the presence of insulin or a phospho-oligosaccharide inhibited the phosphorylation and activation of this enzyme. The inhibitory effect of insulin on the phosphorylation of phospholipid methyltransferase was reversible, and it was mimicked by a phospho-oligosaccharide. The phospho-oligosaccharide was generated by hydrolysis of an isolated glycophospholipid with phosphatidylinositol-specific phospholipase C from Staphylococcus aureus. The insulin-like effect of this phospho-oligosaccharide on the phosphorylation of phospholipid methyltransferase was demonstrated in isolated adipocytes, and the effect was abolished by treatment of the phospho-oligosaccharide with 10% NH4OH, nitrous acid, or sodium periodate. These data suggest that in intact adipocytes the effect of insulin to inhibit the phosphorylation/activation of phospholipid methyltransferase is mediated by a phospho-oligosaccharide generated by a phosphatidylinositol-specific phospholipase C.
Highlights
Addition of isoproterenol to isolated rat adipocytes tion-dephosphorylation in isolated hepatocytes [4,6] and in prelabeled with [32P]phosphatecaused an increase in a subcellular fraction of phospholipid methyltransferase prethe phosphorylation and activation ofphospholipid pared from rat liver microsomes [7]
NH40H, nitrous acid, or sodium periodate. These data lipase C (IO), hasbeendemonstratedtoactin subcellular suggest that in intact adipocytes the effect of insulin to systems to modulate the activities of high affinity phosphoinhibit the phosphorylation/activationof phospholipid methyltransferase is mediated by a phospho-oligosaccharide generated by a phosphatidylinositol-specific phospholipase C
Fat cells were separated into three fractions; P1 contained approximately 15% of the recovered phospholipid methyltransferase activity
Summary
Addition of isoproterenol to isolated rat adipocytes tion-dephosphorylation in isolated hepatocytes [4,6] and in prelabeled with [32P]phosphatecaused an increase in a subcellular fraction of phospholipid methyltransferase prethe phosphorylation and activation ofphospholipid pared from rat liver microsomes [7]. These data lipase C (IO), hasbeendemonstratedtoactin subcellular suggest that in intact adipocytes the effect of insulin to systems to modulate the activities of high affinity phosphoinhibit the phosphorylation/activationof phospholipid methyltransferase is mediated by a phospho-oligosaccharide generated by a phosphatidylinositol-specific phospholipase C. The insulin-like effect of this phospho-oligosaccharide to inhibit hormone-stimulated phospholipid methyltransferase was demonstrated in both a subcellularpreparationandintactadipocytes
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