Separate physiological roles for two isozymes of pyridine nucleotide-linked glycerol-3-phosphate dehydrogenase in...

Two isozymes of diphosphopyridine nucleotide-linked glycerol-3-P dehydrogenase (E.C. 1.1.1.8) were studied with respect to their tissue distribution in chicken, their ontogeny in chicken liver, and their avian taxonomic distribution. These isozymes in chicken are designated "liver type" and "muscle type" based on the tissue of highest concentration.Electrophoretic analysis shows that the liver type is found in small amounts or is undetectable in all tissues studied exept liver. The muscle type is found in skeletal muscles and kidney. Presumptive hybrid enzymes occur at low levels in chicken liver and kidney.The tissue distribution of glycerol-3-P dehydrogenase in several birds capable of sustained flight is different than in chicken. A single electrophoretic form is predominant in both liver and breast muscle with the activity in liver about ten times greater than in breast muscle. Chicken liver and breast muscle extracts have similar levels of glycerol-3-P dehydrogenase activity.Glycerol-3-P dehydrogenase, solely of the liver type, appears simultaneously with liver formation in the chicken embryo and reaches peak concentrations in the liver between the 10th and 14th day of embryonic development. This embryonic pattern is quite different from the muscle type in breast muscle where the enzyme does not appear until after hatching.These observations, when correlated with the metabolism of lipids and carbohydrates in liver and breast muscle of chicken and birds capable of sustained flight, lend support to a hypothesis that the two isozymes have distinct physiological roles. The liver type is associated with the metabolism of the glycerol moiety of triglycerides and phospholipids whereas the muscle type operates in concert with muscle type lactate dehydrogenase to oxidize DPNH during anaerobic muscle glycolysis. The role of glycerol-3-P dehydrogenase in muscle appears to be essential for prolonging anaerobic glycolysis. It is proposed that the isozymes of glycerol-3-P dehydrogenase originated by gene duplication and then diverged via selective evolution to fulfill the metabolic roles proposed.