Glycogen synthase in prenatal rat liver.

Glycogen synthase first appears in significant levels in fetal rat liver on day 18 of gestation (22 day term), but in a mostly inactive, phospho- form. This was reflected in a decreased affinity for its allosteric activator (increased A0.5 for glucose-6-phosphate), and with limited glycogen synthesis and accumulation. Peak glycogen synthesis occurs on day 21 of gestation, and glycogen synthase is in a more dephospho-form with decreased A0.5 for G6P and increased -G6P/+G6P activity ratio. The data suggest that a significant change in phosphorylation state of synthase occurs in late gestation to facilitate the large increase in glycogen synthesis and accumulation.