The nature of the decreased activity of glycogen synthase phosphatase in the liver of the adrenalectomized starved rat

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Abstract
The nature of the decrease in synthase phosphatase activity which occurs progressively in the livers of adrenalectomized rats that are starved for 48 h was investigated. No evidence could be found for the accumulation of an inhibitor. Addition of the heat-stable deinhibitor protein, which antagonizes the effect of thermostable inhibitor proteins (inhibitor-1 and modulator), did not affect the activity of synthase phosphatase in gel-filtered liver extracts from normal or adrenalectomized starved rats; it did, however, increase the activity of phosphorylase phosphatase about 5-fold in either condition. The restoration of synthase phosphatase activity by cortisol in vivo was prevented by actinomycin D. Further evidence concerning the nature of the missing protein came from a comparison of synthase phosphatase activities in liver homogenates from control and adrenalectomized starved rats, with the use of 3 distinct synthase b substrates. The apparent loss of synthase phosphatase activity in the deficient homogenates varied between 30% and 90% according to the type of substrate. The magnitude of this decrease corresponds to the degree of dependence of these substrates on the G-component of synthase phosphatase for efficient conversion to the a-form. No G-component could be isolated from livers of adrenalectomized starved rats. Cross-combination of subcellular fractions from control and deficient livers revealed an almost total loss of G-component, with little loss of S-component. This specific loss of functional G-component is identical to the deficiency previously observed in the livers of rats with severe chronic alloxan-diabetes.