Control of glycogen synthase activity in developing rat liver.

Fasting newborn and growing young rats, though capable of synthesizing liver glycogen when fed, are, unlike adult fasted animals, insensitive to glucocorticoid stimulation of the rate of glucose and lactate incorporation into glycogen. Hormone resistance parallels a decreased liver capability for the synthase b to a conversion reaction up to 2 days after birth, after which the b to a transformation becomes adult type in nature. A comparison of the level of glucose 6-phosphate in liver to the effect of the activator on the synthase activity from newborn rat shows that the enzyme has a greater affinity toward the activator than comparable enzyme from the adult, suggesting the presence of an intermediate metabolite-regulated form of synthase in neonatal liver.