Purification and properties of the major isozymic form of cytoplasmic glycerol-3-phosphate dehydrogenase from rabbit liver

Purification and characterization of human glycerol 3-phosphate dehydrogenases (mitochondrial and cytosolic) by NAD+/NADH redox method

Glycerol 3-phosphate (G3P) shuttle is composed of mGPDH and cGPDH and serves as the interface between carbohydrate- and lipid-metabolism. Recently, these metabolic enzymes have been implicated in type II diabetes mellitus but the detailed kinetic parameters and crystal structure of human mGPDH is unknown, though fewer studies on cGPDH are available. To characterize these enzymes, the human mGPDH and cGPDH genes were optimized and cloned into the pET-SUMO vector and pET-24a(+) vector, respectively, and over-expressed in Escherichia coli BL21 (DE3). However, SUMO-mGPDH was expressed as inclusion bodies. Hence, various culture parameters, solubilizing agents and expression vectors were used to solubilize the protein but they did not produce functional SUMO-mGPDH. Over-expression of SUMO-mGPDH along with molecular chaperone (pG-KJE8) produced a functional SUMO-mGPDH. The functional SUMO-mGPDH was purified and characterized using NAD+/NADH redox method. cGPDH was also over-expressed and purified for its characterization. DLS analysis and CD spectra of the purified proteins were performed. The mGPDH was a monomeric enzyme with MW of ∼74 kDa and displayed optimal activity in the Tris-HCl buffer (pH 7.4); while, cGPDH was a homodimer with a monomeric MW of ∼37 kDa and showed optimal activity in imidazole buffer (pH 8.0). The Kmapp was 0.475 mM for G3P, and 0.734 mM for DHAP. These methods may be used to characterize these enzymes to understand their role in metabolic disorders.

Structural and functional properties of glycerol-3-phosphate dehydrogenase from a mammalian hibernator

Glycerol-3-phosphate dehydrogenase (G3PDH; E.C.1.1.1.8) was purified from liver and skeletal muscle of black-tailed prairie dogs (Cynomys ludivicianus), a hibernating species. Native and subunit molecular masses of the dimeric enzyme were 77 and 40 kD, respectively, and both tissues contained a single isozyme with a pI of 6.4. Kinetic parameters of purified G3PDH from prairie dog liver and muscle were characterized at 22 and 5 °C and compared with rabbit muscle G3PDH. Substrate affinities for hibernator muscle G3PDH were stable (NAD) or increased significantly (K(m) G3P and DHAP decreased) at low temperature whereas K(m) NAD and DHAP of rabbit G3PDH increased. Prairie dog G3PDH showed greater conservation of K(m) G3P over a wide temperature range as well as greater thermal stability and resistance to chemical denaturation by guanidine hydrochloride than the rabbit enzyme. In addition, using the protein sequence of the hibernating thirteen-lined ground squirrel (Ictidomys tridecemlineatus) and bioinformatics tools, the deduced protein structure of G3PDH was compared between heterothermic and homeothermic mammals. Structural and functional characteristics of G3PDH from the hibernating species would support enzyme function over a wide range of core body temperatures over cycles of torpor and arousal.

Direct transfer of NADH between alpha-glycerol phosphate dehydrogenase and lactate dehydrogenase: fact or misinterpretation?

Following the criticism by Chock and Gutfreund [Chock, P.B. & Gutfreund, H. (1988) Proc. Natl. Acad. Sci. USA 85, 8870-8874], that our proposal of direct transfer of NADH between glycerol-3-phosphate dehydrogenase (alpha-glycerol phosphate dehydrogenase, alpha-GDH; EC 1.1.1.8) and L-lactate dehydrogenase (LDH; EC 1.1.1.27) was based on a misinterpretation of the kinetic data, we have reinvestigated the transfer mechanism between this enzyme pair. By using the "enzyme buffering" steady-state kinetic technique [Srivastava, D.K. & Bernhard, S.A. (1984) Biochemistry 23, 4538-4545], we examined the mechanism (random diffusion vs. direct transfer) of transfer of NADH between rabbit muscle alpha-GDH and pig heart LDH. The steady-state data reveal that the LDH-NADH complex and the alpha-GDH-NADH complex can serve as substrate for the alpha-GDH-catalyzed reaction and the LDH-catalyzed reaction, respectively. This is consistent with the direct-transfer mechanism and inconsistent with a mechanism in which free NADH is the only competent substrate for either enzyme-catalyzed reaction. The discrepancy between this conclusion and that of Chock and Gutfreund comes from (i) their incorrect measurement of the Km for NADH in the alpha-GDH-catalyzed reaction, (ii) inadequate design and range of the steady-state kinetic experiments, and (iii) their qualitative assessment of the prediction of the direct-transfer mechanism. Our transient kinetic measurements for the transfer of NADH from alpha-GDH to LDH and from LDH to alpha-GDH show that both are slower than predicted on the basis of free equilibration of NADH through the aqueous environment. The decrease in the rate of equilibration of NADH between alpha-GDH and LDH provides no support for the random-diffusion mechanism; rather, it suggests a direct interaction between enzymes that modulates the transfer rate of NADH. Thus, contrary to Chock and Gutfreund's conclusion, all our experimental data compel us to propose, once again, that NADH is transferred directly between the sites of alpha-GDH and LDH.

Isoenzymic forms of NAD-linked glycerol-3-phosphate dehydrogenase from rabbit brain

Two major enzyme forms of cytosolic NAD-linked glycerol-3-phosphate dehydrogenase in rabbit brain have been purified to apparent homogeneity. One major enzyme form designated I6.5 exhibits an iso-electric point at pH 6.5, and is indistinguishable from the major form I6.5 found in other tissues. The other major form, designated I5.9, has an isolectric point at pH 5.9, and by amino acid analysis is shown to be a true isoenzyme distinct from form I6.5. Form I5.9 appears to be closely related to or identical with the major enzyme characteristic of heart. Neither the brain enzyme form I5.9 nor the major heart isoenzyme are inhibited by antiserum to the muscle enzyme. Because of the high apparent Km for NADH, it is postulated that the brain isoenzyme I5.9 serves to maintain glycolysis when NADH levels rise under relatively anaerobic conditions especially during fetal and neonatal development.

The role of cysteine residues in the catalytic activity of glycerol-3-phosphate dehydrogenase

Glycerol-3-phosphate dehydrogenase (sn-glycerol-3-phosphate:NAD+ 2-oxido-reductase, EC 1.1.1.8) has been shown to be sensitive to inhibition by iodoacetate. The reaction of the enzyme with iodoacetate, which appears to be a simple bimolecular process, is accompanied by a corresponding loss of enzyme activity. In addition to changes in activity, the alkylation reaction was monitored by the incorporation of radioactivity from iodo[2-14C]acetate, by changes in amino acid composition, and by changes in the content of free sulfhydryl groups. It is concluded that there are two cysteine residues in the native dimeric enzyme which are essential for enzymic activity. The rate of inactivation was relatively insensitive to the presence of various compounds with the exception of NADH which markedly inhibited the reaction. Kinetic and binding studies showed that the binding of NADH prevents alkylation and, conversely, alkylation prevents NADH binding. From the pH dependence of the alkylation reaction, the pKa of the essential sulfhydryl groups was calculated to be 8.5 and it is suggested that the binding of coenzyme is independent of the state of ionization of these groups.

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.