L-Glycerol 3-Phosphate Dehydrogenase: I. EFFECTS OF THE SUBSTRATES ON THE CATALYTIC PROPERTIES OF THE HEPATIC NICOTINAMIDE ADENINE DINUCLEOTIDE-LINKED ENZYME FROM THE RABBIT

Abstract

Abstract Effects of the substrate on the initial reaction velocity of nicotinamide adenine dinucleotide-linked l-glycerol 3-phosphate dehydrogenase purified from rabbit liver were studied. The true Michaelis constants for all substrates were determined in 0.1 m tris(hydroxymethyl)aminomethane-HCl buffer (pH 7.5) and were found to be 0.68, 0.19, 0.018, and 0.004 mm for l-glycerol 3-phosphate, nicotinamide adenine dinucleotide, dihydroxyacetone phosphate, and reduced nicotinamide adenine dinucleotide, respectively. The true Michaelis constants measured in 0.1 m glycine-NaOH buffer (pH 10.0) were found to be 0.065 and 0.03 mm for l-glycerol 3-phosphate and nicotinamide adenine dinucleotide, respectively. High concentration of all four substrates in the reaction mixture was found to be to some extent inhibitory. At low concentrations of l-glycerol 3-phosphate or nicotinamide adenine dinucleotide, a plot of initial reaction velocity as a function of l-glycerol 3-phosphate or nicotinamide adenine dinucleotide concentration is sigmoidal. No sigmoidicity is seen in plotting of initial reaction velocity as a function of either dihydroxyacetone phosphate or reduced nicotinamide adenine dinucleotide under the conditions used. However, a plot of initial reaction velocity as a function of dihydroxyacetone phosphate concentration is sigmoidal in the presence of l-glycerol 3-phosphate but is not sigmoidal in the presence of nicotinamide adenine dinucleotide. On the other hand, a plot of initial reaction velocity as a function of reduced nicotinamide adenine dinucleotide concentration is sigmoidal in the presence of nicotinamide adenine dinucleotide but is not sigmoidal in the presence of l-glycerol 3-phosphate. The results suggest that there are two allosteric sites, one for l-glycerol 3-phosphate and the other for nicotinamide adenine dinucleotide. The binding of l-glycerol 3-phosphate to its allosteric site lowers the Michaelis constant for l-glycerol 3-phosphate but increases the Michaelis constant for dihydroxyacetone phosphate. Similarly, the binding of nicotinamide adenine dinucleotide to its allosteric site lowers the Michaelis constant for nicotinamide adenine dinucleotide but increases the Michaelis constant for reduced nicotinamide adenine dinucleotide. Interestingly, the binding of l-glycerol 3-phosphate to its allosteric site does not change the Michaelis constant for either nicotinamide adenine dinucleotide or its reduced form. Similarly, the binding of nicotinamide adenine dinucleotide to its allosteric site does not change the Michaelis constant for either l-glycerol 3-phosphate or dihydroxyacetone phosphate. On the basis of the present study, it is not clear whether dihydroxyacetone phosphate or reduced nicotinamide adenine dinucleotide will bind to the allosteric site for l-glycerol 3-phosphate or nicotinamide adenine dinucleotide, respectively.