Phosphorylation and inactivation of rat hepatocyte glycogen synthase by phorbol esters and mezerein.

Incubation of rat hepatocytes with active phorbol esters and mezerein provoked a decrease in glycogen synthase activity. After the incubation of [3 2 P] phosphate-labeled cells with these tumor promoters, an increase in the amount of 3 2 P bound to the immunoprecipitated enzyme was observed. The decrease in activity highly correlated with the phosphorylation in the smaller CNBr fragment (CB-1) and only at high concentration of the phorbol ester the increase in the phosphorylation of the larger CNBr fragment (CB-2) became significative. Tryptic degradation of CB-1 showed two phosphopeptides after isoelectro focusing analysis (pI 3.9 and pI 3.4) and only one of them (pI 3.9) increased its phosphorylation state after treatment of the cells. These results indicate that the decrease in activity of glycogen synthase by phorbol esters and mezerein is a result of the phosphorylation of the enzyme and that a single site located in CB-1 is preferentially phosphorylated by these agents.