Low Muscle Levels of Pyridoxine in McArdle's Syndrome

Pyridoxal phosphate is a covalently bound cofactor of glycogen phosphorylase. Phosphorylase is a major muscle protein and therefore represents a significant pool of pyridoxal phosphate. Muscle pyridoxine content was measured in three patients with myophosphorylase deficiency (McArdle's syndrome) in whom there was a marked diminution or absence of phosphorylase protein as determined by acrylamide gel electrophoresis. Total muscle pyridoxine in the patients with McArdle's syndrome (0.55 ± 0.08 μg/g wet weight, mean ± SD) was markedly reduced compared with 11 human control subjects who had normal levels of muscle phosphorylase (total muscle B₆ = 2.49 ± 0.47). Despite such drastically low levels of muscle pyridoxine, these patients had no evidence of pyridoxine "deficiency." These results suggest that low muscle B₆ in McArdle's syndrome represents the specific loss of pyridoxal phosphate normally bound to phosphorylase apoenzyme and imply that phosphorylase pyridoxal phosphate accounts for 75 to 80 percent of the total pyridoxine in normal human muscle.