l-Glycerol 3-Phosphate Dehydrogenases

Abstract The effects of sugar phosphates, nucleotides, and other phosphorylated compounds on NAD-linked l-glycerol 3-phosphate dehydrogenase from rabbit liver were studied. Glucose-6-P, fructose-1,6-P2, ribose-5-P, ribulose-1,5-P2, 2,3-propanediol-3-P, and 3-P-glycerate, but not 2-P-glycerate, 2,3-P2-glycerate, or glycerol-2-P, inhibited the oxidation of glycerol-3-P in a noncompetitive manner with respect to either glycerol-3-P or NAD+. The inhibition constants were in the range from 1 to 4 mm. These sugar phosphates in the range from 0.4 to 4 mm did not significantly inhibit the reduction of dihydroxyacetone-P at 0.03 mm dihydroxyacetone-P and 0.01 mm NADH. On the other hand, glyceraldehyde-3-P elicited competitive inhibition (Ki = 0.9 mm) with respect to glycerol-3-P and noncompetitive inhibition (Ki = 1.7 mm) with respect to NAD+. Inorganic phosphate in the range from 1 to 20 mm did not significantly affect the oxidation of glycerol-3-P. However, a significant inhibition on the rate of reduction of dihydroxyacetone-P in a noncompetitive manner with respect to either dihydroxyacetone-P or NADH was observed. Nucleotides and pyrophosphate were found to inhibit both the reverse and forward reactions in a competitive manner with respect to NAD+ or its reduced form and in a noncompetitive manner with respect to glycerol-3-P or dihydroxyacetone-P. From the present kinetic studies, it seems that sugar phosphates, phosphoenolpyruvate, and inorganic phosphate interact with different sites of the enzyme. Nucleotides and pyrophosphate likely interact with the enzyme at the site for NAD+. The physiological significance of the findings in relation to the carbohydrate and lipid metabolism is discussed.