Purification and characterization of a novel form of 20 alpha-hydroxysteroid dehydrogenase from Clostridium scindens.

We have purified a steroid-inducible 20 alpha-hydroxysteroid dehydrogenase from Clostridium scindens to apparent homogeneity. The final enzyme preparation was purified 252-fold, with a recovery of 14%. Denaturing and nondenaturing polyacrylamide gradient gel electrophoresis showed that the native enzyme (Mr, 162,000) was a tetramer composed of subunits with a molecular weight of 40,000. The isoelectric point was approximately pH 6.1. The purified enzyme was highly specific for adrenocorticosteroid substrates possessing 17 alpha, 21-dihydroxy groups. The purified enzyme had high specific activity for the reduction of cortisone (Vmax, 280 nmol/min per mg of protein; Km, 22 microM) but was less reactive with cortisol (Vmax, 120 nmol/min per mg of protein; Km, 32 microM) at pH 6.3. The apparent Km for NADH was 8.1 microM with cortisone (50 microM) as the cosubstrate. Substrate inhibition was observed with concentrations of NADH greater than 0.1 mM. The purified enzyme also catalyzed the oxidation of 20 alpha-dihydrocortisol (Vmax, 200 nmol/min per mg of protein; Km, 41 microM) at pH 7.9. The apparent Km for NAD+ was 526 microM. The initial reaction velocities with NADPH were less than 50% of those with NADH. The amino-terminal sequence was determined to be Ala-Val-Lys-Val-Ala-Ile-Asn-Gly-Phe-Gly-Arg. These results indicate that this enzyme is a novel form of 20 alpha-hydroxysteroid dehydrogenase.     

Coenzyme specificity of mammalian liver D-glycerate dehydrogenase

D-Glycerate dehydrogenase (glyoxylate reductase) was partially purified from rat liver by anion- and cation-exchange chromatography. When assayed in the direction of D-glycerate or glycolate formation, the enzyme was inhibited by high (greater than or equal to 0.5 mM), unphysiological concentrations of hydroxypyruvate or glyoxylate much more potently in the presence of NADPH than in the presence of NADH. However, the dehydrogenase displayed a much greater affinity for NADPH (Km less than 1 microM) than for NADH (Km = 48-153 microM). Furthermore, NADP was over 1000-fold more potent than NAD in inhibiting the enzyme competitively with respect to NADH. NADP also inhibited the reaction competitively with respect to NADPH whereas NAD, at concentrations of up to 10 mM had no inhibitory effect. When measured by the formation of hydroxypyruvate from D-glycerate, the enzyme also displayed a much greater affinity for NADP than for NAD. These properties indicate that liver D-glycerate dehydrogenase functions physiologically as an NADPH-specific reductase. In agreement with this conclusion, the addition of hydroxypyruvate or glyoxylate to suspensions of rat hepatocytes stimulated the pentose-phosphate pathway. The coenzyme specificity of D-glycerate dehydrogenase is discussed in relation to the biochemical findings made in D-glyceric aciduria and in primary hyperoxaluria type II (L-glyceric aciduria).     

Purification and properties of acyl/alkyl dihydroxyacetone-phosphate reductase from guinea pig liver peroxisomes

The peroxisomal acyl/alkyl dihydroxyacetone-phosphate reductase (EC 1.1.1.101) was solubilized and purified 5500-fold from guinea pig liver. The enzyme could be solubilized by detergents only at high ionic strengths in presence of the cosubstrate NADPH. Peroxisomes, isolated from liver by a Nycodenz step density gradient centrifugation, were first treated with 0.2% Triton X-100 to remove the soluble and a large fraction of the membrane-bound proteins. The enzyme was solubilized from the resulting residue by 0.05% Triton X-100, 1 M KCl, 0.3 mM NADPH, and 2 mM dithiothreitol in Tris-HCl buffer (10 mM) at pH 7.5. The enzyme was further purified after precipitating it by dialyzing out the KCl and then resolubilized with 0.8% octyl glucoside in 1 M KCl (plus NADPH and dithiothreitol). The second solubilized enzyme was purified to homogeneity (370-fold from peroxisomes) by gel filtration in a Sepharose CL-6B column followed by affinity chromatography on an NADPH-agarose gel matrix. NADPH-agarose was prepared by reacting periodate-oxidized NADP+ to adipic acid dihydrazide-agarose and then reducing the immobilized NADP+ with NaBH4. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified enzyme showed a single homogeneous band with an apparent molecular weight of 60,000. The molecular weight of the native enzyme was estimated to be 75,000 by size exclusion chromatography. Amino acid analysis of the purified protein showed that hydrophobic amino acid comprised 27% of the molecule. The Km value of the purified enzyme for hexadecyldihydroxyacetone phosphate (DHAP) was 21 microM, and the Vmax value in the presence of 0.07 mM NADPH was 67 mumol/min/mg. The turnover number (Kcat), after correcting for the isotope effect of the cosubstrate NADP3H, was calculated to be 6,000 mol/min/mol of enzyme, assuming the enzyme has a molecular weight of 60,000. The purified enzyme also used palmitoyldihydroxyactone phosphate as a substrate (Km = 15.4 microM, and Vmax = 75 mumol/min/mg). Palmitoyl-DHAP competitively inhibited the reduction of hexadecyl-DHAP, indicating that the same enzyme catalyzes the reduction of both acyl-DHAP and alkyl-DHAP. NADH can substitute for NADPH, but the Km of the enzyme for NADH (1.7 mM) is much higher than that for NADPH (20 microM). The purified enzyme is competitively (against NADPH) inhibited by NADP+ and palmitoyl-CoA. The enzyme is stable on storage at 4 degrees C in the presence of NADPH and dithiothreitol.     

Purification and characterization of a novel NADPH(NADH)-dependent glyoxylate reductase from spinach leaves. Comparison of immunological properties of leaf glyoxylate reductase and hydroxypyruvate reductase.

A novel reductase displaying high specificity for glyoxylate and NADPH was purified 3343-fold from spinach leaves. The enzyme was found to be an oligomer of about 125 kDa, composed of four equal subunits of 33 kDa each. A Km for glyoxylate was about 14-fold lower with NADPH than with NADH (0.085 and 1.10 mM respectively), but the maximal activity, 210 mumol/min per mg of protein, was similar with either cofactor. Km values for NADPH and NADH were 3 and 150 microM respectively. Optimal rates with either NADPH or NADH were found in the pH range 6.5-7.4. The enzyme also showed some reactivity towards hydroxypyruvate with rates less than 2% of those observed for glyoxylate. Results of immunological studies, using antibodies prepared against either glyoxylate reductase or spinach peroxisomal hydroxypyruvate reductase, suggested substantial differences in molecular structure of the two proteins. The high rates of NADPH(NADH)-glyoxylate reductase in crude leaf extracts of spinach, wheat and soya bean (30-45 mumol/h per mg of chlorophyll) and its strong affinity for glyoxylate suggest that the enzyme may be an important side component of photorespiration in vivo. In leaves of nitrogen-fixing legumes, this reductase may also be involved in ureide breakdown, utilizing the glyoxylate produced during allantoate metabolism.     

Partial characterization of steroid sulfohydrolase and steroid sulfotransferase activities in purified porcine Leydig cells

Subcellular fractions of purified pig Leydig cells from 7 different animals have been investigated with respect to their abilities to catalyze the sulfation of several steroids and the hydrolysis of the sulfated forms of these same steroids. Considerable estrone sulfate sulfohydrolase of pH optimum 7.5 and high apparent Km was found to be concentrated in the 105,000 g pellet but no evidence was obtained, in any subcellular fraction, for the presence of any activity toward the 3-sulfate of pregnenolone, dehydroepiandrosterone (DHA) or delta 5-androstene-3 beta,17 beta-diol (androstenediol). Cytosolic sulfotransferase activity toward estrone, pregnenolone, DHA and androstenediol was present in each animal. The activity toward these 4 substrates was eluted from a gel filtration column as a single peak of apparent molecular weight 43 KDa. Upon chromatofocusing, a sharp estrogen sulfotransferase peak of apparent pI 6.1 and pH optimum 9.5, was clearly separated from the neutral steroid sulfotransferase which eluted over a more acidic pH range in a manner suggestive of the presence of several isozymes. This latter, which exhibited a wide pH optimum range between 6 and 8.5, was most active toward androstenediol, and least active toward pregnenolone. The estrogen sulfotransferase exhibited Michaelis-Menten kinetics (apparent Km = 4 microM). The neutral steroid sulfotransferase activity increased in velocity with increasing androstenediol or DHA concentration up to 1 microM beyond which considerable substrate inhibition occurred. It appears from these data that neutral steroid sulfates synthesized in the pig Leydig cell are not subject to enzymic desulfation in the same cells.     

Characterization of prostaglandin E2 20-hydroxylase of sheep vesicular glands

The microsomal fraction of homogenates of the sheep vesicular glands, supplemented with 1 mM NADPH, metabolized 0.2 mM prostaglandin E2 to 20-hydroxyprostaglandin E2 at a rate of 76 +/- 9 pmol/min per mg of protein (with a Km of about 0.1 mM and a Vmax of about 0.1 nmol/min per mg of protein). Prostaglandin E1 was metabolized at a rate of only 8.5% of that of prostaglandin E2. The metabolism of prostaglandin E2 was decreased by 66% using 1 mM NADH instead of NADPH. alpha-Naphthoflavone (50 microM) and carbon monoxide inhibited the 20-hydroxylase by more than 60%, while 1 mM beta-diethylaminoethyl-2,2-diphenyl-pentanoate and 1 mM metyrapone inhibited it by less than 50%. The enzyme catalyzed the incorporation of atmospheric oxygen into the substrate. The findings suggest that the 20-hydroxylase could be a cytochrome P-450. The 20-hydroxylase could not be detected in vesicular glands of five rams 3 weeks after castration. The function of the enzyme is presumably to create the high level of 20-hydroxyprostaglandin E compounds in ram semen.     

2-Aminobenzoyl-CoA monooxygenase/reductase, a novel type of flavoenzyme. Purification and some properties of the enzyme

A novel aerobic mechanism of 2-aminobenzoate metabolism was proposed in a denitrifying Pseudomonas species. 2-Aminobenzoic acid is activated in a coenzyme-A-ligase reaction to 2-aminobenzoyl-CoA and this intermediate is dearomatized by a unique enzyme, tentatively named 2-aminobenzoyl-CoA monooxygenase/reductase. This paper describes the purification and some molecular, kinetic and spectral properties of this flavoenzyme which catalyzes the hydroxylation and reduction of 2-aminobenzoyl-CoA to an unknown non-aromatic compound. 2-Aminobenzoyl-CoA monooxygenase/reductase was purified 25-fold to a specific activity of 25 mumol.min-1.mg-1 protein using ammonium sulfate precipitation, DEAE-cellulose anion-exchange, hydroxylapatite and Mono Q FPLC anion-exchange chromatography. Superose 6 gel filtration for estimation of molecular mass resulted in one symmetrical protein peak corresponding to a molecular mass of 170 kDa. Several experimental data suggest that the protein is probably an alpha 2 dimer; however, it may exist in three dimeric forms, alpha alpha, alpha alpha' and alpha' alpha', where alpha' may be a subunit with a different conformation. Approximately 2 mol noncovalently bound FAD/mol enzyme was found, which in the absence of O2 was reduced by NADH. The enzyme was specific for the substrates 2-aminobenzoyl-CoA (Km less than or equal to 25 microM) and O2 (Km less than or equal to 5 microM), but less specific for the reduced pyridine nucleotides NADH (Km = 42 microM) or NADPH [Km = 500 microM; Vmax (NADH)/Vmax (NADPH) = 1.7:1]. The turnover number was 4250 min-1. The enzyme also reduced N-ethylmaleimide and maleimide with NAD(P)H. The substrate, the products and the reaction stoichiometry are described in two following papers.     

Ecdysone 3-epimerase from the midgut of Manduca sexta (L.).

Ecdysone 3-epimerase was partially purified by ammonium sulfate fractionation from the 100,000 g supernate of Manduca sexta midguts. The enzyme converts ecdysone and 20-hydroxyecdysone to their respective 3-epimers, requires NADH or NADPH and O2 for this reaction, and has the following kinetic parameters: for ecdysone, Km = 17.0 +/- 1.4 microM, Vmax = 110.6 +/- 14.6 pmol min-1 mg-1; for 20-hydroxyecdysone, Km = 47.3 +/- 7.5 microM, Vmax = 131.0 +/- 3.5 pmol min-1 mg-1: for NADPH, Km = 85.4 +/- 10.6 microM; for NADH, Km = 51.3 +/- 1.3 microM. The reaction is irreversible and can be inhibited by various ecdysteroids.     

Inhibition of bovine heart NAD-specific isocitrate dehydrogenase by reduced pyridine nucleotides: modulation of inhibition by ADP, NAD+, Ca2+, citrate, and isocitrate

The activity of NAD-dependent isocitrate dehydrogenase from bovine heart was inhibited by NADH (apparent Ki about 4.3 microM) and NADPH (Ki about 9.8 microM) at subsaturating substrate concentrations at pH 7.4. The inhibition by NADH or NADPH was reversed competitively by magnesium isocitrate in the presence of ADP, but not without ADP. Reversal of inhibition by NADH or NADPH with respect to NAD+ was competitive or of the linear mixed type depending on whether ADP was absent or present. ADP3- (0.2 mM) increased the Ki(app) for NADPH from 9.8 to 27.1 microM; further addition of Ca2+ (0.2 mM) raised the Ki(app) to 127 microM. For the modification of NADPH inhibition by ADP, S0.5 for Ca2+ was approximately 48 microM. This compares to the Km for Ca2+ of 0.3-1 microM for the activation of the enzyme without NADPH [Denton, R. M., Richards, D. A., & Chin, J. G. (1978) Biochem. J. 176, 899-906; Aogaichi, T., Evans, J., Gabriel, J., & Plaut, G. W. E. (1980) Arch. Biochem. Biophys. 204, 350-360]. ADP did not affect the Ki for NADH. Magnesium citrate, which was about 100-fold more effective as a positive modifier of the enzyme with ADP than without ADP [Gabriel, J. L., & Plaut, G. W. E. (1983) Fed. Proc., Fed. Am. Soc. Exp. Biol. 42, 2082], reversed competitively the inhibition by NADPH in the presence of ADP, but not without ADP. Magnesium citrate did not reverse NADH inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification and properties of biliverdin reductases from pig spleen and rat liver.

Biliverdin reductase was purified from pig spleen soluble fraction to a purity of more than 90% as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme was a monomer protein with a molecular weight of about 34,000. Its isoelectric point was at 6.1-6.2. The enzyme was strictly specific to biliverdin and no other oxiodoreductase activities could be detected in the purified enzyme preparation. The purified enzyme could utilize both NADPH and NADH as electron donors for the reduction of biliverdin. However, there were considerable differences in the kinetic properties of the NADPH-dependent and the NADH-dependent biliverdin reductase activities: Km for NADPH was below 5 microM while that for NADH was 1.5-2 mM; the pH optimum of the reaction with NADPH was 8.5 whereas that of the reaction with NADH was 6.9; Km for biliverdin in the NADPH system was 0.3 microM whereas that in the NADH system was 1-2 microM. In addition, both the NADPH-dependent and NADH-dependent activities were inhibited by excess biliverdin, but this inhibition was far more pronounced in the NADPH system than in the NADH system. IX alpha-biliverdin was the most effective substrate among the four biliverdin isomers, and the dimethylester of IX alpha-biliverdin could not serve as a substrate. Biliverdin reductase was also purified about 300-fold from rat liver soluble fraction. The hepatic enzyme was also a monomer protein with a molecular weight of 34,000 and showed properties quite similar to those of the splenic enzyme as regards the biliverdin reductase reaction. The isoelectric point of the hepatic enzyme, however, was about 5.4. It was assumed that NADPH rather than NADH is the physiological electron donor in the intracellular reduction of IX alpha-biliverdin. The stimulatory effects of bovine and human serum albumins on the biliverdin reductase reactions were also examined.     

Purification and characterization of hydroxypyruvate reductase from the facultative methylotroph Methylobacterium extorquens AM1.

Hydroxypyruvate reductase was purified to homogeneity from the facultative methylotroph Methylobacterium extorquens AM1. It has a molecular mass of about 71 kDa, and it consists of two identical subunits with a molecular mass of about 37 kDa. This enzyme uses both NADH (Km = 0.04 mM) and NADPH (Km = 0.06 mM) as cofactors, uses hydroxypyruvate (Km = 0.1 mM) and glyoxylate (Km = 1.5 mM) as the only substrates for the forward reaction, and carries out the reverse reaction with glycerate (Km = 2.6 mM) only. It was not possible to detect the conversion of glycolate to glyoxylate, a proposed role for this enzyme. Kinetics and inhibitory studies of the enzyme from M. extorquens AM1 suggest that hydroxypyruvate reductase is not a site for regulation of the serine cycle at the level of enzyme activity.     

3 alpha-hydroxysteroid dehydrogenase activity catalyzed by purified pig adrenal 20 alpha-hydroxysteroid dehydrogenase.

In earlier studies, two distinct molecules, 20 alpha-HSD-I and 20 alpha-HSD-II, responsible for 20 alpha-HSD activity of pig adrenal cytosol were purified to homogeneity and characterized [S. Nakajin et al., J. Steroid Biochem. 33 (1989) 1181-1189]. We report here that the purified 20 alpha-HSD-I, which mainly catalyzes the reduction of 17 alpha-hydroxyprogesterone to 17 alpha,20 alpha-dihydroxy-4-pregnen-3-one, catalyzes 3 alpha-hydroxysteroid oxidoreductase activity for 5 alpha (or 5 beta)-androstanes (C19), 5 alpha (or 5 beta)-pregnanes (C21) in the presence of NADPH as the preferred cofactor. The purified enzyme has a preference for the 5 alpha (or 5 beta)-androstane substrates rather than 5 alpha (or 5 beta)-pregnane substrates, and the 5 beta-isomers rather than 5 alpha-isomers, respectively. Kinetic constants in the reduction for 5 alpha-androstanedione (Km; 3.3 microM, Vmax; 69.7 nmol/min/mg) and 5 beta-androstanedione (Km; 7.7 microM, Vmax; 135.7 nmol/min/mg) were demonstrated for comparison with those for 17 alpha-hydroxyprogesterone (Km; 26.2 microM, Vmax; 1.3 nmol/min/mg) which is a substrate for 20 alpha-HSD activity. Regarding oxidation, the apparent Km and Vmax values for 3 alpha-hydroxy-5 alpha-androstan-17-one were 1.7 microM and 43.2 nmol/min/mg, and 1.2 microM and 32.1 nmol/min/mg for 3 alpha-hydroxy-5 beta-androstan-17-one, respectively. 20 alpha-HSD activity in the reduction of 17 alpha-hydroxyprogesterone catalyzed by the purified enzyme was inhibited competitively by addition of 5 alpha-DHT with a Ki value of 2.0 microM. Furthermore, 17 alpha-hydroxyprogesterone inhibited competitively 3 alpha-HSD activity with a Ki value of 150 microM.     

Purification and properties of NADH/NADPH-dependent p-hydroxybenzoate hydroxylase from Corynebacterium cyclohexanicum

Crude soluble extracts of Corynebacterium cyclohexanicum, grown on cyclohexanecarboxylic acid, were found to contain 4-hydroxybenzoate 3-hydroxylase which functions with NADH as well as NADPH. The purified enzyme preparation was electrophoretically homogeneous and contained FAD as prosthetic group. The relative molecular mass of the enzyme was estimated to be about 47000 by native and denaturated acrylamide gel electrophoresis, indicating that it is monomeric. The enzyme was stable at 60 degrees C for 10 min. The enzyme was highly specific for p-hydroxybenzoate. The activity was inhibited by several aromatic analogues of p-hydroxybenzoate such as p-aminobenzoate, p-fluorobenzoate, o-hydroxybenzoate, m-hydroxybenzoate, 2,4-dihydroxygenzoate, and 2,5-dihydroxybenzoate. The Km value for NADH was fairly constant, about 45 microM, in the pH range 7.0-8.4, whereas the Km value for NADPH increased from 63 microM to 170 microM as the pH rose from 7.0 to 8.4. V values in the same pH range, however, were approximately constant in both cases; about 30 mumol min-1 mg-1 for NADH, and 26 mumol min-1 mg-1 for NADPH. Mg2+ was required for full activity of the enzyme in low concentrations of phosphate buffer. The enzyme was inhibited by C1- which was non-competitive with respect to NADH, NADPH and p-hydroxybenzoate.     

Cell-free mercury volatilization activity from three marine caulobacter strains

Three mercury-resistant marine Caulobacter strains showed an inducible mercury volatilization activity. Cell-free mercury volatilization (mercuric reductase) from these three marine Caulobacter strains was characterized and compared with enzyme activities determined by plasmids of Escherichia coli and Staphylococcus aureus. The temperature sensitivity of the Caulobacter mercuric reductase was greater than that of mercuric reductase from other gram-negative sources. Cell-free enzyme activity required NADH or NADPH, with NADPH functioning much better at lower concentrations than NADH. The Km for the Caulobacter enzyme was 4 microM Hg2+. Ag+ was a competitive inhibitor of Caulobacter mercuric reductase (Ki = 0.2 microM Ag+), as with previously studied enzymes. Arsenite was a noncompetitive inhibitor of the Caulobacter enzyme with a Ki of 75 microM AsO2-.     

Cell-free mercury volatilization activity from three marine caulobacter strains.

Three mercury-resistant marine Caulobacter strains showed an inducible mercury volatilization activity. Cell-free mercury volatilization (mercuric reductase) from these three marine Caulobacter strains was characterized and compared with enzyme activities determined by plasmids of Escherichia coli and Staphylococcus aureus. The temperature sensitivity of the Caulobacter mercuric reductase was greater than that of mercuric reductase from other gram-negative sources. Cell-free enzyme activity required NADH or NADPH, with NADPH functioning much better at lower concentrations than NADH. The Km for the Caulobacter enzyme was 4 microM Hg2+. Ag+ was a competitive inhibitor of Caulobacter mercuric reductase (Ki = 0.2 microM Ag+), as with previously studied enzymes. Arsenite was a noncompetitive inhibitor of the Caulobacter enzyme with a Ki of 75 microM AsO2-.     

Novel prostaglandin dehydrogenase in rat skin.

Present evidence suggests that skin is an important organ of prostaglandin metabolism. To clarify its role, the basic kinetics of 15-hydroxyprostaglandin dehydrogenase (PGDH) from rat skin were investigated with either NAD+ of NADP+ as co-substrate. Prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E2 (PGE2) were used as substrates and preliminary studies were made of the inhibitory effects of the reduced co-substrates NADH and NADPH. A radiochemical assay was used in which [3H]PGF2 alpha or [14C]PGE2 were incubated with high-speed supernatant of rat skin homogenates. The substrate and products were then extracted by solvent partition, separated by t.l.c. and quantified by liquid-scintillation counting. At linear reaction rates and at an NAD+ concentration of 10 mM the mean apparent Km for PGF2 alpha was 24 microM with a mean apparent Vmax. of 9.8 nmol/s per litre of reaction mixture. For PGE2 the mean apparent Km was 8 microM, with a mean apparent Vmax, of 2.7 nmol/s per litre of reaction mixture. With NADP+ as a co-substrate at a concentration of 5 mM a mean apparent Km of 23 microM was obtained for PGF2 alpha with a mean apparent Vmax. of 5.2 nmol/s per litre. For PGE2 values of 7.5 microM and 3.0 nmol/s per litre were obtained respectively. These results show that skin contains NAD+- and NADP+-dependent PGDH. An important finding was that the NADP+-linked enzyme gave Km values for PGE2 that were considerably lower than those reported for NADP+-linked PGDH from other tissues. Furthermore, preliminary inhibition studies with the NAD+-linked PGDH system indicate that this enzyme is not only inhibited by NADH, but also by NADPH, a property not previously reported for NAD+-linked PGDH derived from other tissues.     

Partial purification, substrate specificity and regulation of alpha-L-glycerolphosphate dehydrogenase from Saccharomyces carlsbergensis.

alpha-L-Glycerolphosphate dehydrogenase (sn-glycerol-3-phosphate:NAD+ 2-oxidoreductase, EC 1.1.1.8) from Saccharomyces carlsbergensis was purified 400-fold. The enzyme preparation is free of interfering activities, such as glyceraldehyde phosphate dehydrogenase, alcohol dehydrogenase, triose phosphate isomerase and glycerolphosphatase. At pH 7.0 it is specific for NADH (Km = 0.027 mM with 0.8 mM dihydroxyacetone phosphate) and dihydroxyacetone phosphate (Km = 0.2 mM with 0.2 mM NADH). Between pH 5.0 and 6.0 the enzyme functions with NADPH, but only at 7% of the rate with NADH. Various anions (I- greater than SO42- greater than Br- greater than Cl-) act as inhibitors competing with the substrate dihydroxyacetone phosphate. Inorganic phosphate (Ki = 0.1 mM), pyrophosphate and arsenate are strong inhibitors. The nucleotides ATP and ADP are also inhibitory, but their action seems to be of the same type as the general anion competition (Ki = 0.73 mM for ATP). The results are consistent with the notion that the enzyme may regulate the redox potential of the NAD+/NADH couple during fermentation.     

Inhibition of hepatic adenylate cyclase by NADH

Adenylate cyclase activity in isolated rat liver plasma membranes was inhibited by NADH in a concentration-dependent manner. Half-maximal inhibition of adenylate cyclase was observed at 120 microM concentration of NADH. The effect of NADH was specific since adenylate cyclase activity was not altered by NAD+, NADP+, NADPH, and nicotinic acid. The ability of NADH to inhibit adenylate cyclase was not altered when the enzyme was stimulated by activating the cyclase was not altered when the enzyme was stimulated by activating the Gs regulatory element with either glucagon or cholera toxin. Similarly, inhibition of Gi function by pertussis toxin treatment of membranes did not attenuate the ability of NADH to inhibit adenylate cyclase activity. Inhibition of adenylate cyclase activity to the same extent in the presence and absence of the Gpp (NH) p suggested that NADH directly affects the catalytic subunit. This notion was confirmed by the finding that NADH also inhibited solubilized adenylate cyclase in the absence of Gpp (NH)p. Kinetic analysis of the NADH-mediated inhibition suggested that NADH competes with ATP to inhibit adenylate cyclase; in the presence of NADH (1 mM) the Km for ATP was increased from 0.24 +/- 0.02 mM to 0.44 +/- 0.08 mM with no change in Vmax. This observation and the inability of high NADH concentrations to completely inhibit the enzyme suggest that NADH interacts at a site(s) on the enzyme to increase the Km for ATP by 2-fold and this inhibitory effect is overcome at high ATP concentrations.     

Inhibition of the methylcoenzyme M methylreductase system by NAD+ and NADP+ in cell-extracts of Methanosarcina barkeri

In cell extracts of Methanosarcina barkeri, the methylcoenzyme M methylreductase system with H2 as the electron donor was inhibited by NAD+ and NADP+, but NADH and NADPH had no effect on enzyme activity. NAD+ (4 and 8 mM) shifted the saturation curve for methylcoenzyme M from hyperbolic (Hill coefficient [nH] = 1.0; concentration of substrate giving half maximal velocity [Km] = 0.21 mM) to sigmoidal (nH = 1.5 and 2.0), increased Km (Km = 0.25 and 0.34 mM), and slightly decreased Vmax. Similarly NADP+ at 4m and 8 mM increased nH to 1.6 and 1.85 respectively, but the Km values (0.3 and 0.56 mM) indicated that NADP+ was a more efficient inhibitor than NAD+.     

Purification and characterization of a 15-ketoprostaglandin delta 13-reductase from bovine lung.

15-Ketoprostaglandin delta 13-reductase from bovine lung has been purified using affinity chromatography to apparent homogeneity, as judged from polyacrylamide gel electrophoresis with and without sodium dodecyl sulphate. Valine was identified as tne N-terminal aumino acid, and the isoelectric point was estimated at pH 7.8. Molecular weights of 56,000 and 39,500 were found by the use of gel filtration and SDS-polyacrylamide gel electrophoresis, respectively. The enzyme was found to be specific for the 15-keto group, thus 15-ketoprostaglandin E4 (apparent Km = microM) is a substrate, in contrast to prostaglandin E1. The enzyme was active with both NADH (apparent Km = 88--94 microM) and NADH (apparent Km = 5--9 microM) as coenzyme, but the V max with NADH was more than twice that obtained with NADPH. The enzyme did not catalyze the reversed reaction: 13,14-dihydro-15-keto-prostaglandin E1 to 15-ketoprostaglandin E1. The turnover number of the enzyme was determined to be either 60 or 42 min-1. The low value of the turnover number is compensated by a high concentration (96.4 mU/g tissue) of the enzyme in lung tissue, resulting in a high metabolic capacity. Thus, 15-ketoprostaglandin delta 13-reductase together with 15-hydroxyprostaglandin dehydrogenase ensures an irreversible catabolism of prostaglandins.