Pyridoxal phosphate, the cofactor of glycogen phosphorylase, fulfils the criteria needed of a turnover label for this enzyme. The decay of protein-bound label following administration of [3H]pyridoxine is a good index of the rate of degradation of the enzyme in vivo. This method has been applied to the study of catabolism of the enzyme in normal, denervated and dystrophic mouse skeletal muscle. In both of the pathological conditions the enzyme is degraded more rapidly than normal.

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