No limiting role for glycogenin in determining maximal attainable glycogenlevels in rat skeletal muscle

We examined whether the protein level and/or activity of glycogenin, the pr otein core upon which glycogen is synthesized, is limiting for maximal atta inable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 +/- 1.4, 34.7 +/- 1.7, and 39.7 +/- 1.3 mU/mg protein in white gastrocnemi us, red gastrocnemius, and soleus muscles, respectively. A similar fiber ty pe dependency of glycogenin protein levels was seen. Neither glycogenin pro tein level nor the activity of glycogenin correlated with previously determ ined maximal attainable glycogen levels, which were 69.3 +/- 5.8, 137.4 +/- 10.1, and 80.0 +/- 5.4 mu mol/g wet wt in white gastrocnemius, red gastroc nemius, and soleus muscles, respectively. In additional experiments, rats w ere exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles (similar to 25% ). This increase in maximal glycogen levels was not accompanied by an incre ase in glycogenin protein level or activity. Furthermore, even in the prese nce of very high glycogen levels (similar to 170 mu mol/g wet wt), similar to 30% of the total glycogen pool continued to be present as unsaturated gl ycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skelet al muscle.