Circadian rhythm in mouse liver phosphorylase activity

Studies of the liver glycogen phosphorylase activity in mice at different times of the day have made it possible to describe circadian variations in the activity of this enzyme. These variations are closely related with the circadian variations in the glycogen content, since the highest value of the enzyme activity is reached at the time (16.00 hr) when the glycogen is maximally degradated and vice‐versa. The lowest value of phosphorylase activity was found between 24.00 hr and 4.00 hr. The difference between the peak and the trough was highly significant: p < 0.001. The estimation of the enzyme activity was performed on liver homogenates by the method described by Sutherland. The mechanisms which may be involved in the control of this rhythm are discussed. It is suggested that this rhythm may be the result of synthesis and degradation rather than activation and inactivation of the enzyme.