Glycogen phosphorylase metabolism turnover

There maybe multiple metabolic pools of the vitamin, with very different rates of turnover. In this case, short-term and long-term studies will give very different estimates of the fractional rate of turnover of the toted body pool. As discussed in Section 9.6.1, this is known to be a problem with vitamin Be, because some 80% of the total body pool is associated with muscle glycogen phosphorylase and has a much lower fractional turnover rate than the remedning 20%. 

This is considerably lower than the requirements estimated from depletion/ repletion studies (Section 9.6.2) and may reflect dilution of the small pool associated with amino acid metabolism, which has a rapid turnover, by the larger and more stable pool associated with glycogen phosphorylase. 

FIG. 1. Interrelationships between vitamin B6 and phosphorylase metabolism. The low rate of turnover of glycogen phosphorylase (gpb) and the lack of exchange of free and protein-bound PLP mean that exchange into the muscle pool is largely controlled by the kinetics of turnover of the enzyme. At present, it is not known whether resolution of the holo-enzyme is a prerequisite or consequence of phosphorylase degradation. Reproduced with permission of llie Biochemical Journal. 

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