Determination of molybdenum and tungsten in biological materials

In vitro complementation of assimilatory NAD(P)H-nitrate reductase from mutants of Chlamydomonas reinhardii

In vitro complementation of the soluble assimilatory NAD(P)H-nitrate reductase (NAD(P)H:nitrate oxidoreductase, EC 1.6.6.2) was attained by mixing cell-free preparations of Chlamydomonas reinhardii mutant 104, uniquely possessing nitrate-inducible NAD(P)H-cytochrome c reductase, and mutant 305 which possesses solely the nitrate-inducible FMNH2- and reduced benzyl viologen-nitrate reductase activities. Full activity and integrity of NAD(P)H-cytochrome c reductase from mutant 104 and reduced benzyl viologen-nitrate reductase from mutant 305 are needed for the complementation to take place. A constitutive and heat-labile molybdenum-containing cofactor, that reconstitutes the NAD(P)H-nitrate reductase activity of nit-1 Neurospora crassa but is incapable of complementing with 104 from C. reinhardii, is present in the wild type and 305 algal strains. The complemented NAD(P)H-nitrate reductase has been purified 100-fold and was found to be similar to the wild enzyme in sucrose density sedimentation, molecular size, pH optimum, kinetic parameters, substrate affinity and sensitivity to inhibitors and temperature. From previous data and data presented in this article on 104 and 305 mutant activities, it is concluded that C. reinhardii NAD(P)H-nitrate reductase is a heteromultimeric complex consisting of, at least, two types of subunits separately responsible for the NAD(P)H-cytochrome c reductase and the reduced benzyl viologen-nitrate reductase activities.

The influence of tungsten on the molybdenum status of poultry

1. Large doses of tungsten, administered to the chick either by injection or by feeding, increased tissue concentrations of tungsten and decreased tissue concentrations of molybdenum and tissue activities of xanthine dehydrogenase. 2. The rate of loss of large doses of tungsten from the liver occurred in an exponential manner with a half-life of 27 h. 3. When tungsten was administered to chicks fed on a semi-synthetic diet containing abnormally low concentrations of molybdenum, the activity of hepatic xanthine dehydrogenase was reduced to negligible levels. 4. The alterations in molybdenum metabolism resulting from the administration of large doses of tungsten to the chick appears to be the result of tungsten toxicity and not of molybdenum deficiency. 5. Deaths from tungsten toxicity occurred when tissue concentrations of tungsten were increased to approximately 25 micrograms/g liver. At this tissue tungstencon centration the activity of xanthine dehydrogenase was zero.

Purification and properties of nitrogenase from the cyanobacterium, Anabaena cylindrica

The nitrogenase complex was isolated from nitrogen-starved cultures of Anabaema cylindrica. Sodium dithionite, photochemically reduced ferredoxin, and NADPH were found to be effective election donors to nitro genase in crude extracts whereas hydrogen and pyruvate were not. The Km for acetylene in vivo is ten-fold higher than the Km in vitro, whereas this pattern does not hold for the non-heterocystous cyanobacterium, Plectonema boryanum. This indicates that at least one mechanism of oxygen protection in vivo involves a gas diffusion barrier presented by the heterocyst cell wall. The Mo-Fe component was purified to homogeneity. Its molecular weight (220,000), subunit composition, isoelectric point (4.8), Mo, Fe, and S2- content (2, 20 and 20 mol/mol component), and amino acid composition indicate that this component has similar properties to Mo-Fe-containing components isolated from other bacterial sources. The isolated components from A. cylindrica were found to cross-react, to varying degrees, with components isolated from Azotobacter vinelandii, Rhodospirillum rubrum, and P. boryanum.

The formate dehydrogenase involved in electron transport from formate to fumarate in Vibrio succinogenes

1. The formate dehydrogenase of Vibrio succinogenes, which is involved in electron transport with fumarate as terminal acceptor, was solubilized with Triton X-100 and purified some 200-fold by means of chromatography on hydroxyapatite, sucrose-density-gradient centrifugation and chromatography on DEAE-Sephadex. Gel filtration failed to increase the specific acitivity of the enzyme while gel electrophoresis in the presence of dodecylsulfate revealed that 73% of the protein of the preparation consisted of a polypeptide of Mr 110 000. The Mr of the functional enzyme was found to be 263 000 on the basis of the Stokes radius (5.8 nm) and the sedimentation coefficient (11.3 S). 2. The preparation contained 9 micronmol molybdenum/g protein and about 170 mumol iron-sulfur/g protein. The contents of b and c cytochromes varied and were lower than that of molybdenum. The low-potential cytochrome b [Kröger, A. and Innerhofer, A. (1976) Eur. J. Biochem. 69, 497-506] present in the preparation was reduced by formate. 3. The preparation catalyzed the reduction of a variety of dyes by formate, but not of NAD, FMN, ferredoxin or oxygen. The reduction of CO2 or bicarbonate by reduced methyl viologen was not catalyzed. The reaction with benzyl viologen obeyed the rate law consistent with a ping-pong mechanism. The Km for formate was 1.5 mM at infinite concentration of benzyl viologen while that for benzyl viologen was 0.53 mM at infinite formate concentration. Enzymic activity was inhibited by azide, KCN and HgCl2, but not by 4-chloromercuriphenylsulfonate or 2-(n-nonyl)-4-hydroxyquinoline-N-oxide, both of which inhibit overall electron transport. The inhibition by azide was competitive with formate; the Ki was 45 micron. 4. The midpoint potential of the low-potential cytochrome b of the membrane fraction was shifted -40 mV by the presence of 2-(n-nonyl)-4-hydroxyquinoline-N-oxide. 5. It is concluded that the formate dehydrogenase of V. succinogenes is isolated as a dimer consisting of two identical subunits of Mr 110,000, each of which carries one atom of molybdenum and iron-sulfur groups. The low-potential cytochrome b is the direct acceptor for the electrons of formate dehydrogenase in the electron transport of formate-fumarate reduction of V. succinogenes. Inhibition of electron transport of the membrane fraction between formate dehydrogenase and menaquinone by 2-(n-nonyl)-4-hydroxyquinoline-N-oxide [Kröger, A. and Innerhofer, A. (1976) Eur. J. Biochem. 69, 487-495] is caused by the inhibitor binding to the low-potential cytochrome b.

Purification and characterization of homogeneous assimilatory reduced nicotinamide adenine dinucleotide phosphate-nitrate reductase from Neurospora crassa

Neurospora crassa wild type STA4 NADPH-nitrate reductase (NADPH : nitrate oxidoreductase, EC 1.6.6.3) has been purified 5000-fold with an overall yield of 25--50%. The final purified enzyme contained 4 associated enzymatic activities: NADPH-nitrate reductase, FADH2-nitrate reductase, reduced methyl viologen-nitrate reductase and NADPH-cytochrome c reductase. Polyacrylamide gel electrophoresis yielded 1 major and 1 minor protein band and both bands exhibited NADPH-nitrate and reduced methyl viologen-nitrate reductase activities. SDS gel electrophoresis yielded 2 protein bands corresponding to molecular weights of 115 000 and 130 000. A single N-terminal amino acid (glutamic acid) was found and proteolytic mapping for the two separated subunits appeared similar. Purified NADPH-nitrate reductase contained 1 mol of molybdenum and 2 mol of cytochrome b557 per mol protein. Non-heme iron, zinc and copper were not detectable. It is proposed that the Neurospora assimilatory NADPH-nitrate reductase consists of 2 similar cytochrome b557-containing 4.5-S subunits linked together by one molybdenum cofactor. A revised electron flow scheme is presented. p-Hydroxymercuribenzoate inhibition was reversed by sulfhydryl reagents. Inhibitory pattern of p-hydroxymercuribenzoate and phenylglyoxal revealed accessible sulfhydryl and arginyl residue(s) as functional group(s) in the earlier part of electron transport chain as possibly the binding site of NADPH or FAD.

Microbial fixation of nitrogen in presence of sodium tungstate

The effect of sodium tungstate in the culture media of three different species of Azotobacter, isolated from Allahabad soil, was studied. It was observed that the presence of tungstate in the culture media of bacterial sample A2 and A3 decreased the fixation of nitrogen, except in the bacterial sample A1.

Regulation of molybdate transport by Clostridium pasteurianum

The regulation of the molybdate (MoO42-) transport activity of Clostridium pasteurianum has been studied by observing the effects of NH3, carbamyl phosphate, MoO42-, and chloramphenicol on the ability of cells to take up MoO42-. Compared with cells fixing N2, cells grown in the presence of 1 mM NH3 are greater than 95% repressed for MoO42- transport. Uptake activity begins to increase just before NH exhaustion (under Ar or N2) and continues to increase throughout the lag period as cells shift from NH3-growing to N2-fixing conditions. When cells are shifted from N2-fixing to NH3-growing conditions the transport activity per fixed number of cells decreases by increase of bells in absence of transport synthesis. Carbamyl phosphate (greater than or equal to 15 mM) but not NH3 inhibits 58% of the in vitro uptake activity. When 1 mM carbamyl phosphate is added just before the exhaustion of NH3, the transport activity, measured 2 h later, is 100% repressed. Cells grown in the presence of high MoO42- (1mM) are 80% repressed for MoO42- transport. Synthesis of the MoO42- transport system is also completely stopped when chloramphenicol (300 mug/ml) is added just before the exhaustion oNH 3 from the medium. These findings demonstrate that the ability of cells to transport MoO42- is dependent upon new protein synthesis and can be repressed by high levels of substrate. The regulation of MoO42- uptake by NH3 or carbamyl phosphate closely parallels the regulation of nitrogenase activity. Activity of neither nitrogenase component (Fe protein or MoFe protein) was detected even 3 h after the exhaustion of the NH3 if either MoO42- was absent or if WO42- was present in place of MoO42-. The duration of the diauxic lag increases with decreasing concentration of MoO42- in the medium. If no MoO42- is present the lag continues indefinitely. If MoO42- is added late in the lag period, growth under N2-fixing conditions resumes but only after a normal induction period.

Role of molybdenum in dinitrogen fixation by Clostridium pasteurianum

The role of Mo in the activity and synthesis of the nitrogenase components of Clostridium pasteurianum has been studied by observing the competition of Mo with its structural analogue W. Clostridial cells when fixing N2 appeared strictly dependent upon the available Mo, showing maximal N2-fixing activity at molybdate concentrations in the media of 10 muM. Cells grown in media with 3 times 10(-6) muM Mo, although showing good growth, had only 15% as much N2-fixing activity. In the presence of W the synthesis of both nitrogenase components, molybdoferredoxin and azoferredoxin, was affected. Attempts to produce nitrogenase in W-grown cells by addition of high molybdenum to the media in the presence of inhibitors of protein synthesis showed that Mo incorporation into a possible inactive preformed apoenzyme did not occur. Unlike other molybdoenzyme-containing cells, in which W either is incorporated in place of Mo to yield inactive protein or initiates the production of apoprotein, C. pasteurianum forms neither a tungsten substituted molybdoferredoxin nor an apoprotein. It is concluded that in C. pasteurianum molybdenum is an essential requirement for both the biosynthesis and activity of its nitrogenase.