INOSITOL GLYCAN PHOSPHATE DERIVED FROM HUMAN ERYTHROCYTE ACETYLCHOLINESTERASE GLYCOLIPID ANCHOR AND INOSITOL CYCLIC 1,2-PHOSPHATE ANTAGONIZE GLUCAGON ACTIVATION OF GLYCOGEN-PHOSPHORYLASE

In this study we examine the hypothesis that an inositol glycan phosph ate can act similarly to insulin on intact cells. The inositol glycan phosphate used in this study (glycan alpha) was isolated previously fr om the glycoinositol phospholipid anchor of human erythrocyte acetylch olinesterase and was shown to have the structure anolamine-PO4)Man-(N, N-dimethyl)GlcN-inositol-PO4. The cellular response investigated was t he glucagon-stimulated activation of glycogen phosphorylase in rat hep atocytes. When hepatocytes were incubated with 20 nM glucagon for 4 mi n, the ratio of phosphorylase a activity to total phosphorylase increa sed from a basal value of 0.49 +/- 0.02 to 0.82 +/- 0.03 (means +/- SE , n = 15). Inclusion of either 100 nM insulin or 3-10 muM glycan alpha during the glucagon incubation significantly decreased the glucagon-s timulated activity ratio to 0.74 +/- 0.03 for either agent. Furthermor e, hepatocyte preparations differed in their response to insulin and w ere divided into insulin-responsive and -resistant groups. Glycan alph a had a significant effect only in the insulin-responsive group for wh ich the observed activity ratio for 10 muM glycan alpha plus glucagon (0.68 +/- 0.05) compared closely with that for insulin plus glucagon ( 0.70 +/- 0.04). For the insulin-resistant group, the activity ratio in the presence of 10 muM glycan alpha was 0.81 +/- 0.03, unchanged from the control with glucagon alone. Because glycan alpha contains an ino sitol phosphate group, the effect of inositol cyclic 1,2-phosphate on the glucagon-stimulated activity ratio was determined. inositol cyclic 1,2-phosphate at 1-10 muM significantly decreased the activity ratio to the same extent as insulin in seven insulin-responsive preparations . Glycan alpha and inositol cyclic 1,2-phosphate differed from insulin in eliciting no decrease in the 8-bromoadenosine cyclic 3'-5'-monopho sphate-stimulated phosphorylase activity ratio in hepatocytes. The res ults indicate that glycan alpha and inositol cyclic 1,2-phosphate can mimic some effects of insulin on intact hepatocytes.