Comparison of cytoplasmic glycerol-3-phosphate dehydrogenase from rat liver and muscle

Cytoplasmic NAD-linked l-glycerol-3-P dehydrogenase (EC 1.1.1.8) appears as a single major zone of enzymic activity when homogenates from skeletal muscle of rats weighing 120-200 g (10-14 weeks) are examined by polyacrylamide gel isoelectric focusing. Muscle homogenates from rats weighing 250-350 g (16-24 weeks) generally showed two major zones of glycerol-3-P dehydrogenase activity on electrofocused gels. Homogenates from the younger, smaller rats which initially had only one major form of the enzyme were able to generate the second and more acidic form of the enzyme on standing for several days at 4°. Rat liver and muscle glycerol-3-P dehydrogenases did not resolve from one another when mixed homogenates were analyzed by gel electrofocusing and by disc electrophoresis. Apparently homogeneous liver and muscle enzyme preparations also failed to resolve from one another on cellulose acetate electrophoresis, although both could be resolved from the rabbit muscle enzyme. The amino acid compositions of rat liver and muscle glycerol-3-P dehydrogenases are very similar but appear to differ in their contents of glutamic acid-glutamine and of methionine. Evaluation of the significance of these differences requires confirmatory analysis of the rat muscle enzyme.