Turnover of skeletal muscle glycogen phosphorylase

Glycogen phosphorylase is a major sarcoplasmic protein and is one of the key regulatory enzymes in the control of glycogen utilisation. In C57BL/6J mice, the activity of the enzyme decreases under muscle-wasting conditions, including denervation-induced atrophy and muscular dystrophy. The cofactor of this enzyme, pyridoxal phosphate, has been employed as a specific label to permit measurement of the rate of degradation of the enzyme in vivo. In both of the muscle-wasting conditions, the decay of protein-bound label occurs at a higher rate than in normal animals, suggesting accelerated degradation of the enzyme. Additionally, we have used a monoclonal antibody, specific for pyridoxal phosphate, as a sensitive probe to search for degradation intermediates of this enzyme in vivo.