A phosphooligosaccharide has been proposed as a second messenger of insulin. It is believed to be structurally related to the carbohydrate moiety of phosphatidylinositol glycan anchors of many cell surface proteins. Herein we demonstrate that [32]phosphate in freshly isolated adipocytes and [3H]galactose in cultured hepatoma cells (H4IIE) labeled the same set of three different glycolipids. With all three, the radiolabel was made water soluble by phosphatidylinositol(glycan)-specific phospholipase C or D catalyzed hydrolysis. We isolated the three phospholipase C-released substances. One of them was susceptible to nitrous acid deamination, indicative of a hexosamine with a free amino group. This phosphooligosaccharide structure had an apparent molecular mass between tetra- and pentaglucose by gel filtration. By anion-exchange chromatography it was separated into two differently charged and interconvertible species. Adipocytes stimulated with insulin accumulated the nitrous acid sensitive phosphooligosaccharide: after stimulation the intracellular level of free phosphooligosaccharide increased threefold within 5 min, fell off during the next few minutes and then remained at a slightly elevated level. After insulin stimulation the intracellular concentration of free phosphooligosaccharide was > 1,000-fold higher than in the incubation medium. When prepared from rat livers on a preparative scale, the oligosaccharide was also found to exhibit insulinomimetic effects on protein phosphorylation of insulin target proteins in intact adipocytes. After subcellular fractionation of adipocytes the lipid-bound [32P]phosphooligosaccharide of the plasma membrane was found to be localized in plasma membrane domains apparently corresponding to caveolae. Lipid-bound [32P]phosphooligosaccharide was found also in the microsomal fraction.

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