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1 Department of Human Biology and Nutritional Science, University of Guelph, Guelph, ON, Canada
2 Institute of Exercise and Sport Sciences, Copenhagen Muscle Research Centre, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: jshearer{at}ucalgary.ca.
Glycogenin is the self-glycosylating protein primer that initiates glycogen granule formation. To examine the role of this protein during glycogen resynthesis, eight, male subjects exercised to exhaustion on a cycle ergometer at 75% VO2 max followed by 5 x 30s sprints at maximal capacity to further deplete glycogen stores. During recovery, carbohydrate (75g/h) was supplied to promote rapid glycogen repletion and muscle biopsies were obtained from the vastus lateralis at 0, 30, 120 and 300min post-exercise. At time 0, no free (deglycosylated) glycogenin was detected in muscle indicating all glycogenin was complexed to carbohydrate. Glycogenin activity, a measure of the glycosylating ability of the protein increased at 30min and remained elevated for the remainder of the study. Quantitative RT-PCR showed elevated glycogenin mRNA at 120min followed by increases in protein levels (as determined by immunoblotting) at 300min. Glycogenin specific activity (glycogenin activity/relative protein content) was also elevated at 120min. Proglycogen increased at all time points with the highest rate of resynthesis occurring between 0-30min. In comparison, macroglycogen levels did not significantly increase until 300min post-exercise. Together, these results show that during recovery from prolonged exhaustive exercise, glycogenin mRNA, protein content and activity increase in muscle. This may facilitate rapid glycogen resynthesis by providing the glycogenin backbone of proglycogen, the major component of glycogen synthesized in early recovery.
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