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 phosphate can act similarly to insulin on intact cells. The inositol glycan phosphate used in this study (glycan alpha) was isolated previously from the glycoinositol phospholipid anchor of human erythrocyte acetylcholinesterase and was shown to have the structure glycine-ethanolamine-PO4-Man-Man-(N,N-dimethylethanolamine-PO4)Man- (N,N-dimethyl)GlcN-inositol-PO4. The cellular response investigated was the glucagon-stimulated activation of glycogen phosphorylase in rat hepatocytes. When hepatocytes were incubated with 20 nM glucagon for 4 min, the ratio of phosphorylase a activity to total phosphorylase increased from a basal value of 0.49 +/- 0.02 to 0.82 +/- 0.03 (mean +/- SE, n = 15). Inclusion of either 100 nM insulin or 3-10 microM glycan alpha during the glucagon incubation significantly decreased the glucagon-stimulated activity ratio to 0.74 +/- 0.03 for either agent. Furthermore, hepatocyte preparations differed in their response to insulin and were divided into insulin-responsive and -resistant groups. Glycan alpha had a significant effect only in the insulin-responsive group for which the observed activity ratio for 10 microM 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 microM glycan alpha was 0.81 +/- 0.03, unchanged from the control with glucagon alone. Because glycan alpha contains an inositol phosphate group, the effect of inositol cyclic 1,2-phosphate on the glucagon-stimulated activity ratio was determined.(ABSTRACT TRUNCATED AT 250 WORDS)