The results of in vivo and in vitro activation of mouse liver glycogen phosphorylase at two different times of the day are reported. Phosphorylase activity measured at 4 p.m. cannot be further increased over the basal level (40 umol Pi. min^-1, g^-1 fresh liver) either in vivo (epinephrine) or in vitro(ATP-Mg incubation). On the contrary, at midnight the basal level (18 umol Pi. min^-1, g^-1 fresh liver) can be increased by 50% by epinephrine in vivo, or brought to 30 umol Pi. min^-1.g^-1 fresh liver by incubation with ATP-Mg of homogenates from either control or epinephrineinjected mice. These results suggest that conversion from the inactive to the active form can account for approximately half of the daily variations in phosphorylase activity. The remaining variation might be due to other factors, e.g. de novo synthesis and degradation.

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