Structural studies on neonatal rat liver glycogen synthase: a comparison between adult and newborn synthase phosphopeptides.

Liver glycogen synthase has been isolated from newborn rats and phosphorylated in vitro with the catalytic subunit of cAMP-dependent protein kinase. The isolated newborn synthase b is dependent upon Glc 6-P for activity, like adult synthase b, but has a high affinity toward Glc 6-P, unlike adult synthase b but like adult synthase a. Phosphorylation decreases the newborn synthase affinity toward Glc 6-P to the same value as adult synthase b. A comparison of adult and newborn synthase 32Pi-labeled trypsin and chymotrypsin peptide fragments by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that the newborn synthase has structural properties significantly different from the adult enzyme. Thus, a fetal isozyme of synthase in the newborn rat could account, in part, for the difference in catalytic properties, relative to adult synthase.