Nonpolar Interactions of Inhibitors with the Nicotinamide Adenine Dinucleotide-binding Sites of L-α-Glycerophosphate Dehydrogenase

Isolation and characterization of adipose tissue glycerol-3-phosphate dehydrogenase

Recent studies on the differentiation of human preadipocytes have extensively used GPDH as a convenient marker enzyme to follow the development of the cells. Since the properties of human adipose tissue GPDH is largely unknown, it was considered of interest to characterize the purified enzyme. Glycerol-3-phosphate dehydrogenase was purified to homogeneity using blue Sepharose and hydroxylapatite chromatography. Monomeric molecular mass of GPDH was estimated using SDS-PAGE while the dimeric mass was estimated using non-denaturing PAGE. Fluorometric titrations were used to measure the binding of NADH to the enzyme. Inactivation experiments with proteolytic enzymes, urea and heat treatment were used to investigate a possible conformational change due to NADH binding. The purified enzyme displayed a monomeric molecular mass of 35,000 Da, a dimeric molecular mass of 74,000 Da and an isoelectric point (pI) of 5.85. The enzyme exhibited a pH optimum of 7.5 for the reduction of DHAP and 9.0 for G-3-P oxidation. Glycerol (50%) was found to stabilize the enzyme activity during storage, but altered the kinetic properties of the enzyme, acting as a competitive activator with respect to DHAP reduction. GPDH was inhibited by sulfhydryl modifying reagents and fatty acids. The effectiveness of inhibition by saturated fatty acids increased proportionately with chain length from decanoate to stearate. In addition preincubation of the enzyme, in the presence of oleate, resulted in a time dependent inactivation. Time dependent inactivation of GPDH by both iodoacetate and oleate was prevented by the presence of NADH but not NAD+.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of the coenzyme binding site of rat ovarian 20 alpha-hydroxysteroid dehydrogenase

Rat ovarian 20 alpha-hydroxysteroid dehydrogenase was shown to be effectively inhibited by adenosine derivatives, nicotinamide derivatives, NADP analogs, N-alkylammonium chlorides, and carboxylic acids through coenzyme-competitive inhibition studies. Multiple inhibition analysis was used to demonstrate either simultaneous binding of inhibitors that interact with different regions of the NADP-binding site or mutual exclusion of inhibitors that interact with the same region on the enzyme. The results of these studies demonstrated that the 2'-phosphate, the pyrophosphate, and the positively charged ring nitrogen are important features of the coenzyme structure in binding to the coenzyme-binding site of the enzyme. In addition, the presence of a hydrophobic region near the NADP-binding site was indicated by positive chainlength effects observed in the binding of nicotinamide derivatives, alkylammonium chlorides, and carboxylic acids.

The interaction of fatty acids with rabbit liver and muscle glycerol-3-phosphate dehydrogenase

Various fatty acids containing 10--22 carbons and including unsaturated derivatives were found to be inhibitors of rabbit liver and skeletal muscle sn-glycerol-3-phosphate dehydrogenase (sn-glycerol-3-phosphate:NAD+ 2-oxidoreductase, EC 1.1.1.8). For the liver enzyme, the logarithm of the inhibition constant was linearly related to the number of carbon atoms in the saturated fatty acids whereas the muscle enzyme, which was generally more strongly inhibited, showed a nonlinear dependence. The liver and muscle enzymes also interacted differently with a series of unsaturated fatty acids for which a high degree of specificity was exhibited which was related to the position, configuration, and number of double bonds in the compound. A steady-state kinetic analysis shows that under some conditions, the kinetics of the NADH reduction of dihydroxyacetone phosphate by NADH in the presence of stearic acid do not follow simple Michaelis-Menten behavior but rather the velocity shows a sigmoidal dependence on fatty acid concentration and strong substrate inhibition. Stearic acid is a much poorer inhibitor of the NAD-dependent oxidation of glycerol-3-phosphate. At low substrate concentrations stearic acid is competitive with respect to NAD with an inhibition constant of 24 micrometer for stearic acid. In addition to the effect of fatty acids on the initial velocities of the enzyme-catalyzed reactions, preincubation of the enzyme with fatty acid leads to a slow, time-dependent irreversible inactivation of the enzyme which is prevented by the presence of NADH. The results are discussed in terms of the differences in the conformations of the hydrophobic binding sites on the two enzymes.