Qualitative change of GTP-binding protein.

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In order to know the qualitative changes of inhibitory GTPbinding(G)protein in vitro, we did ADP-ribosylation of Gi protein by pertussis toxin(islet-activating protein, IAP), an experiment about dissociation of the trimer of the three alpha beta gamma-subunits into alpha -subunits and beta gamma -subunits and western blot using an antibody for Gi-protein. The addition with lithium ion to the protein caused a decrease of the amounts of ADPribosylation of Gi protein by IAP in human platelet membranes and weekened the inhibition'of adenylate cyclase activity by activated Gi-protein, though the dissociation of the three subunits did not change. Also, chronic treatment of manic depressive patients with lithium carbonate decreased the amounts of ADPribosylation by IAP in their platelet membranes. Next the effect of phosphorylation of partially purified Gi-protein by cyclic AMP-dependent protein kinase was studied. The treatment caused a decrease of the amounts of ADP-ribosylation of Gi protein by IAP in human platelet membranes, weekened the phospholipase C activity by activated Gi-protein in differentiated HL-60 cells and inhibited the dissociation of the three subunits induced by Mg2+ and GTPgamma S. Western blot showed no remarkable quantitative changes in Gi protein in the experimental conditions mentioned above. The results strongly suggested that the qualitative changes of Gi-proteins are physiologically and pathologically functional in the regulation of transmembrane signal transduction.