Glycogenin activity in human skeletal muscle is proportional to muscle glycogen concentration

The de novo biosynthesis of glycogen is catalyzed by glycogenin, a self-glu cosylating protein primer. To date, the role of glycogenin in regulating gl ycogen metabolism and the attainment of maximal glycogen levels in skeletal muscle are unknown. We measured glycogenin activity after enzymatic remova l of glucose by alpha-amylase, an indirect measure of glycogenin amount. Se ven male subjects performed an exercise and dietary protocol that resulted in one high-carbohydrate leg (HL) and one low-carbohydrate leg (LL) before testing. Resting muscle biopsies were obtained and analyzed for total glyco gen, proglycogen (PG), macroglycogen (MG), and glycogenin activity. Results showed differences (P < 0.05) between HL and LL for total glycogen (438.0 +/- 69.5 vs. 305.7 +/- 57.4 mmol glucosyl units/kg dry wt) and PG (311.4 +/ - 38.1 vs. 227.3 +/- 33.1 mmol glucosyl units/kg dry wt). A positive correl ation between total muscle glycogen content and glycogenin activity (r = 0. 84, P < 0.001) was observed. Similar positive correlations (P < 0.05) were also evident between both PG and MG concentration and glycogenin activity ( PG, r = 0.82; MG, r = 0.84). It can be concluded that glycogenin does displ ay activity in human skeletal muscle and is proportional to glycogen concen tration. Thus it must be considered as a potential regulator of glycogen sy nthesis in human skeletal muscle.