A Phospho-oligosaccharide Mimics the Effect of Insulin to Inhibit Isoproterenol-dependent Phosphorylation of Phospholipid Methyltransferase in Isolated Adipocytes

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.