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At least 19 recordsLinked to original sources
PubMed2000

Mutations in the molybdenum cofactor biosynthetic protein Cnx1G from Arabidopsis thaliana define functions for molybdopterin binding, molybdenum insertion, and molybdenum cofactor stabilization.

The molybdenum cofactor (Moco), a highly conserved pterin compound coordinating molybdenum (Mo), is required for the enzymatic activities of molybdoenzymes. In all organisms studied so far Moco is synthesized by a unique and evolutionary old multistep pathway that requires the activities of at least six gene products. In eukaryotes, the last step of Moco synthesis, i.e., transfer and insertion of Mo into molybdopterin (MPT), is catalyzed by the two-domain proteins Cnx1 in plants and gephyrin in mammals. Both domains (E and G) of these proteins are able to bind MPT in vitro. Here, we show the identification and mutational dissection of functionally important regions within the Cnx1 G domain that are essential for MPT binding, the conversion of MPT to Moco, and Moco stabilization. By functional screening for mutants in the Cnx1 G domain that are no longer able to complement Escherichia coli mogA mutants, we found two classes of mutations in highly conserved amino acid residues. (i) The first class affects in vitro binding of MPT to the protein and the stabilization of Moco, the product of the G domain. (ii) The second class is represented by two independent mutations in the aspartate 515 position that is not affected in MPT binding and Moco stabilization; rather the conversion of MPT to Moco by using bound MPT and a yet unknown form of Mo is completely abolished. The results presented here provide biochemical evidence for a purified Cnx1 G domain catalyzing the insertion of Mo into MPT.

Amino Acid Sequence↗
PubMed1993

Molybdenum-independent nitrogenases of Azotobacter vinelandii: a functional species of alternative nitrogenase-3 isolated from a molybdenum-tolerant strain contains an iron-molybdenum cofactor.

Nitrogenase-3 of Azotobacter vinelandii is synthesized under conditions of molybdenum and vanadium deficiency. The minimal metal requirement for its synthesis, and its metal content, indicated that the only transition metal in nitrogenase-3 was iron [Chisnell, Premakumar and Bishop (1988) J. Bacteriol. 170, 27-33; Pau, Mitchenall and Robson (1989) J. Bacteriol. 171, 124-129]. A new species of nitrogenase-3 has been purified from a strain of A. vinelandii (RP306) lacking structural genes for the Mo- and V-nitrogenases and containing a mutation which enables nitrogenase-3 to be synthesized in the presence of molybdenum. SDS/PAGE showed that component 1 contained a 15 kDa polypeptide which N-terminal amino acid sequence determination showed to be encoded by anfG. This confirms that nitrogenase-3, like V-nitrogenase, comprises three subunits. Preparations of the nitrogenase-3 from strain RP306 contained 24 Fe atoms and 1 Mo atom per molecule. Characterization of the cofactor centre of the enzyme by e.p.r. spectroscopy and an enzymic cofactor assay, together with stimulation of the growth of strain RP306 by Mo, showed that nitrogenase-3 can incorporate the Mo-nitrogenase cofactor (FeMoco) to form a functional enzyme. The specific activities (nmol of product produced/min per mg of protein) determined from activity titration curves were: under N2, NH3 formation 110, with concomitant H2 evolution of 220; under argon, H2 evolution 350; under 10% acetylene (C2H2) in argon, ethylene (C2H4) 58, ethane (C2H6) 26, and concomitant H2 evolution 226. The rate of formation of C2H6 was non-linear, and the C2H6/C2H4 ratio strongly dependent on the ratio of nitrogenase components.

Amino Acids↗
PubMed1999

Molybdenum absorption and utilization in humans from soy and kale intrinsically labeled with stable isotopes of molybdenum.

BACKGROUND: Stable-isotope studies of molybdenum metabolism have been conducted in which molybdenum was added to the diet and was assumed to be absorbed and utilized similarly to the molybdenum in foods. OBJECTIVE: Our objective was to establish whether the molybdenum in foods is metabolized similarly to molybdenum added to the diet. DESIGN: We first studied whether sufficient amounts of molybdenum stable isotopes could be incorporated into wheat, kale, and soy for use in a human study. Enough molybdenum could be incorporated into soy and kale to study molybdenum absorption and excretion. Two studies were then conducted, one in women and one in men. In the first study, each meal contained approximately 100 microg Mo from soy, kale, and extrinsic molybdenum. In the second study, soy and extrinsic molybdenum were compared; the meal contained approximately 300 microg Mo. RESULTS: In the first study, molybdenum was absorbed equally well from kale and an extrinsic source. However, the molybdenum in soy was less well absorbed than the molybdenum in kale or that added to the diet. In the second study, absorption of molybdenum from soy was less than from the extrinsic label. Urinary excretion of soy molybdenum was also lower than urinary excretion of the extrinsic label, but excretion as a percentage of the absorbed dose was not significantly different between treatments. CONCLUSIONS: The molybdenum in soy is less available than molybdenum added to the diet, but the molybdenum in kale is as available as molybdenum added to the diet. Once absorbed, excretion is not significantly different for soy, kale, and extrinsic molybdenum.

Adult↗
PubMed1995

Molybdenum absorption, excretion, and retention studied with stable isotopes in young men at five intakes of dietary molybdenum.

A study of molybdenum absorption, excretion, and balance was conducted in four young men fed five amounts of dietary molybdenum, ranging from 22 to 1490 micrograms/d, for 24 d each. The study was conducted to obtain scientific data on which to base a recommendation on dietary molybdenum intake for healthy young men. Stable isotopes of molybdenum were used as tracers. 100Mo was fed five times during the study and 97Mo was infused three times. 94Mo was used to quantify the molybdenum isotopes and total molybdenum in urine, fecal collections, and diets by isotope dilution. Adverse effects were not observed at any of the dietary intakes. Molybdenum was very efficiently absorbed, 88-93%, at all dietary molybdenum intakes, and adsorption was most efficient at the highest amounts of dietary molybdenum. The amount and percentage of molybdenum excreted in the urine increased as dietary molybdenum increased, suggesting that molybdenum turnover is slow when dietary molybdenum is low and increases as dietary molybdenum increases. We conclude from these results that dietary intakes between 22 and 1500 micrograms/d by adult men are safe for > or = 24 d and that molybdenum retention is regulated by urinary excretion. Molybdenum is conserved at low intakes and excess molybdenum is rapidly excreted in the urine when intake is high.

Adult↗
PubMed1998

Interaction of Sulfate-Reducing Bacteria with Molybdenum Dissolved from Sputter-Deposited Molybdenum Thin Films and Pure Molybdenum Powder.

When sputter-deposited Mo thin films were exposed to sulfate-reducing bacterium Desulfovibrio desulfuricans, dissolved Mo markedly delayed the culture growth and reduced the rate of sulfate reduction. The interaction led to an orange coloration of the culture liquid. X-ray photoelectron spectroscopy of dried culture droplets revealed that Mo dissolution products existed mostly in pentavalent state, and a smaller amount of molybdate and molybdenum disulfide. In contrast, Mo dissolution in uninoculated medium was negligible. Subsequently, different concentrations of molybdate, ranging from 0.1 to 20 mM, were added to the growth medium and it was found that a low concentration of molybdate (1 mM) was able to reduce the culture growth rate and sulfate reduction by forming Mo(V)-S complexes. In order to study the dependence of the degree of interaction upon microbial activity and growth-dependent metabolic products, 1.0 g/L Mo powder was added to (a) the growth medium, (b) a 3-day-old culture, and, (c) the supernatants of 2 h to 5-day-old cultures. Ultraviolet-visible spectroscopy indicated that the Mo(V)-S complexes consisted of a Mo-S compound analogous to a binuclear dioxobridged Mo(V)-cysteine complex (314 nm) and Mo(V)-containing molybdenyl thiocyanate (468 nm). Dissolution of Mo was induced by H2S, a product of the bacterial sulfate reduction, and was further increased probably by sulfur-containing amino groups and proteins. Copyright 1998 Academic Press.

Journal Article↗
PubMed2004

Plasma molybdenum reflects dietary molybdenum intake.

The relationship between plasma molybdenum (Mo) and dietary intake has not been investigated in humans. We developed an isotope dilution method to determine molybdenum in 0.5 mL blood plasma by ICP-MS and conducted a study to determine the effect of dietary intake on plasma molybdenum. Twelve young men consumed a very low Mo diet (22 microg/day) for 24 days while confined to the WHNRC metabolic research unit and plasma molybdenum was monitored. (97)Mo was infused in four of the subjects (Group 1) to follow its clearance from the blood. The other eight remained in unit for 120 days (an additional 96 days). Four consumed the 22 microg/day molybdenum diet for 102 days followed by 467 microg/day for 18 days (Group 2). and four consumed five levels of dietary molybdenum for 24 days each (Group 3). (100)Mo was added to the diet one or more times at each dietary level. Total plasma molybdenum and (100)Mo were monitored throughout the study. Plasma molybdenum in the 12 subjects decreased from 8.2 +/- 0.5 to 6.1 +/- 0.5 nmol/L after 13 days of low molybdenum intake and was 5.1 +/- 0.5 nmol/L after 24 days. In Group 2, average plasma molybdenum was 7.8 +/- 0.9 nmol/L at the beginning of the study, 5.4 +/- 0.4 nmol/L during the 102 days low molybdenum period, and 16.5 +/- 0.6 nmol/L during the high molybdenum period. Plasma molybdenum in Group 3 was 4.2 +/- 2.1 nmol/L at 22 microg/day; 5.8 +/- 2.5 nmol/L at 72 microg/day; 6.6 +/- 2.3 nmol/L at 121 microg/day; 19.7 nmol/L +/-2.1 at 467 microg/day; and 43.9 +/- 2.1 nmol/L at 1490 microg/day. The results demonstrate that, in contrast to most other essential minerals, plasma molybdenum reflects low and high dietary molybdenum intakes within 14 days and may a useful indicator of low and high dietary intakes.

Adult↗
PubMed1985

Effects of copper sulfate supplement on growth, tissue concentration, and ruminal solubilities of molybdenum and copper in sheep fed low and high molybdenum diets.

Each of four groups of six wethers were fed one of a low molybdenum, high molybdenum, low molybdenum plus copper sulfate, or high molybdenum plus copper sulfate corn silage-based diet for ad libitum intake for 221 days. Average daily gains and ratios of feed/gain were depressed for the high molybdenum diet as compared with the low molybdenum diet suggesting molybdenum toxicity in sheep fed the high molybdenum diet. This was alleviated partly by the copper sulfate supplement. The supplement also decreased solubility of both copper and molybdenum in the rumen but had no effect on copper concentration in blood plasma. Concentration of molybdenum was higher in both liver and kidney in sheep fed high-molybdenum diets as compared with low-molybdenum diets. Copper concentration was higher in kidneys of sheep fed high-molybdenum diets, but no difference was significant in liver copper between sheep fed diets high or low in molybdenum.

Animals↗
PubMed2006

Molybdenum kinetics in men differ during molybdenum depletion and repletion.

In this study we developed an expanded compartmental model of molybdenum (Mo) kinetics to determine rates of molybdenum distribution during molybdenum depletion and repletion. The model was based on a clinical study in which 4 men consumed a low-molybdenum diet of 22 microg/d (0.23 micromol/d) for 102 d, followed by a high molybdenum diet of 467 microg/d (4.9 micromol/d) for 18 d. Stable isotopes 100Mo and 97Mo were administered orally and intravenously, respectively, at several time points during the study, and serial samples of plasma, urine, and feces were analyzed for 100Mo, 97Mo, and total Mo. Based on plasma, urine, and fecal molybdenum levels, kinetic parameters of distribution and elimination were determined. The rates of molybdenum distribution and elimination were different during depletion and repletion. During high intake, urinary molybdenum excretion was greater than during low intake. In addition, fractional tissue storage of molybdenum was lower during high intake than during low intake. This suggests that low intake results in an adaptation to conserve body Mo, and that high intake results in an adaptation to eliminate Mo. The model also suggested that food-bound molybdenum was approximately 16% less bioavailable than purified Mo. Finally, under the conditions of this study, the model suggested that an intake of 43 microg/d (0.45 micromol/d) would be sufficient to maintain plasma molybdenum levels at steady state. This is a minimum estimate because subjects in this study were in a molybdenum-sparing state. These findings provide an understanding of the adaptations in molybdenum metabolism that take place during depletion and repletion.

Adult↗
PubMed1976

Molybdenum toxicity: interactions between copper, molybdenum and sulphate.

Molybdenum toxicity and the interactions between copper, molybdenum and sulphate are reviewed. The main signs of molybdenum poisoning are poor growth and anaemia (rat, chick, rabbit, cattle and sheep), anorexia (rat), diarrhoea and achromotrichia (cattle and sheep), joint and bone deformities (rat, rabbit, cattle), central nervous system degeneration and loss of crimp in wool (sheep). The following topics are discussed: (1) The effect of sulphate and sulphur compounds on molybdenum toxicity. (2) The effect of molybdenum on tissue copper levels. (3) The effect of molybdenum on the distribution of copper in plasma. (4) The effect of molybdenum on uptake and excretion of copper. (5) The possible existence of copper(II) molybdate in vivo. (6) The influence of molybdenum on sulphide production by ruminal micro-organisms. (7) Competition between molybdenum and sulphate in intestinal transport. (8) Interaction of sulphur with copper in vivo. (9) The possible involvement of molybdenum in gout and multiple sclerosis in humans.

Animals↗
PubMed1985

Biosynthesis of the iron-molybdenum cofactor and the molybdenum cofactor in Klebsiella pneumoniae: effect of sulfur source.

NifQ- and Mol- mutants of Klebsiella pneumoniae show an elevated molybdenum requirement for nitrogen fixation. Substitution of cystine for sulfate as the sulfur source in the medium reduced the molybdenum requirement of these mutants to levels required by the wild type. Cystine also increased the intracellular molybdenum accumulation of NifQ- and Mol- mutants. Cystine did not affect the molybdenum requirement or accumulation in wild-type K. pneumoniae. Sulfate transport and metabolism in K. pneumoniae were repressed by cystine. However, the effect of cystine on the molybdenum requirement could not be explained by an interaction between sulfate and molybdate at the transport level. Cystine increased the molybdenum requirement of Mol- mutants for nitrate reductase activity by at least 100-fold. Cystine had the same effect on the molybdenum requirement for nitrate reductase activity in Escherichia coli ChlD- mutants. This shows that cystine does not have a generalized effect on molybdenum metabolism. Millimolar concentrations of molybdate inhibited nitrogenase and nitrate reductase derepression with sulfate as the sulfur source, but not with cystine. The inhibition was the result of a specific antagonism of sulfate metabolism by molybdate. The effects of nifQ and mol mutations on nitrogenase could be suppressed either by the addition of cystine or by high concentrations of molybdate. This suggests that a sulfur donor and molybdenum interact at an early step in the biosynthesis of the iron-molybdenum cofactor. This interaction might occur nonenzymatically when the levels of the reactants are high.

Cysteine↗
PubMed2002

Bacterial expression of the molybdenum domain of assimilatory nitrate reductase: production of both the functional molybdenum-containing domain and the nonfunctional tungsten analog.

Assimilatory NADH:nitrate reductase (EC 1.6.6.1), a complex molybdenum-, cytochrome b(557)- and FAD-containing protein, catalyzes the regulated and rate-limiting step in the utilization of inorganic nitrogen by higher plants. To facilitate structure/function studies of the individual molybdenum center, we have developed bacterial expression systems for the heterologous production of the 541 residue amino-terminal, molybdenum center-containing domain of spinach nitrate reductase either as a six-histidine-tagged variant or as a glutathione-S-transferase-tagged fusion protein. Expression of the his-tagged molybdenum domain in Escherichia coli BL21(DE3) cells under anaerobic conditions yielded a 55-kDa domain with a specific activity of 1.5 micromol NO(3)(-) consumed/min/nmol enzyme and with a K(mapp)(NO(3)(-)) of 8 mciroM. In contrast, expression of the molybdenum domain as a GST-tagged fusion protein in E. coli TP1000(MobA(-) strain) cells under aerobic conditions yielded an 85-kDa fusion protein with a specific activity of 10.8 micromol NO(3)(-) consumed/min/nmol enzyme and with a K(mapp)(NO(3)(-)) of 12 microM. Fluorescence analysis indicated that both forms of the molybdenum domain contained the cofactor, MPT, although the MPT content was higher in the GST-fusion domain. Inductively coupled plasma mass spectrometric analysis of both the his-tagged and GST-fusion protein domain samples indicated Mo/protein ratios of 0.44 and 0.93, respectively, confirming a very high level of Mo incorporation in the GST-fusion protein. Expression of the GST-fusion protein in TP1000 cells in the presence of elevated tungsten concentrations resulted in an 85-kDa fusion protein that contained MPT but which was devoid of nitrate-reducing activity. Partial reduction of the molybdenum domain resulted in the generation of an axial Mo(V) EPR species with g values of 1.9952, 1.9693, and 1.9665, respectively, and exhibiting superhyperfine coupling to a single exchangeable proton, analogous to that previously observed for the native enzyme. In contrast, the tungsten-substituted MPT-containing domain yielded a W(V) EPR species with g values of 1.9560, 1.9474, and 1.9271, respectively, with unresolved superhyperfine interaction. NADH:nitrate reductase activity could be reconstituted using the GST-molybdenum domain fusion protein in the presence of the recombinant forms of the spinach nitrate reductase' flavin- and heme-containing domains.

Catalytic Domain↗
PubMed1996

Promoters controlling expression of the alternative nitrogenase and the molybdenum uptake system in Rhodobacter capsulatus are activated by NtrC, independent of sigma54, and repressed by molybdenum.

The alternative nitrogenase of Rhodobacter capsulatus is expressed only under conditions of nitrogen and molybdenum depletion. The analysis of anfA-lacZ fusions demonstrated that this dual control occurred at the level of transcription of anfA, which encodes a transcriptional activator specific for the alternative nitrogenase. The anfA promoter was found to be activated under nitrogen-limiting conditions by NtrC in a sigma54-independent manner. In addition, anfA transcription was repressed by traces of molybdenum. This molybdenum-dependent repression of anfA was released in R. capsulatus mutants carrying either lesions in the high-affinity molybdenum uptake system (modABCD) or a double deletion of mopA and mopB, two genes encoding molybdenum-pterin-binding proteins. The expression of the molybdenum transport system itself was shown to be negatively regulated by molybdenum and, unexpectedly, to be also regulated by NtrC. This finding is in line with the presence of two tandemly arranged DNA motifs located in front of the R. capsulatus mopA-modABCD operon, which are homologous to R. capsulatus NtrC binding sites. Mapping of the transcriptional initiation sites of mopA and anfA revealed promoter sequences exhibiting significant homology to each other but no homology to known prokaryotic promoters. In addition, a conserved DNA sequence of dyad symmetry overlapping the transcriptional initiation sites of mopA and anfA was found. Deletions within this element resulted in molybdenum-independent expression of anfA, indicating that this DNA sequence may be the target of MopA/MopB-mediated repression.

Bacterial Proteins↗
PubMed1996

Molybdenum metabolism in men with increasing molybdenum intakes: changes in kinetic parameters.

Molybdenum metabolism was studied in four young men to determine the effect of the amount of dietary molybdenum on molybdenum kinetics. A compartmental model, developed by using data from a study with low dietary molybdenum, was adapted to accommodate five levels of molybdenum. Each level, ranging from 22 to 1,470 micrograms molybdenum/day, was fed for 24 days. Kinetics of absorption and excretion were traced by using 97Mo (intravenous) and 100 Mo (oral) stable-isotope tracers at selected intervals. Urinary and fecal isotope excretion data for 6-day pooled collections were fit to a kinetic model by using SAAM/CONSAM software. Residence times for molybdenum were estimated at 2.4 days in the gastrointestinal tract, 40 min in plasma, from 3.3 to 0.3 days in fast-turnover tissue, and from 63 to 237 days in slow-turnover tissue. As dietary molybdenum increased, residence time decreased in fast-turnover tissue and increased in slow-turnover tissue. The model closely approximated the highly efficient homeostatic mechanisms of molybdenum metabolism over a wide range of intakes.

Adult↗
PubMed1996

Kinetic model of molybdenum metabolism developed from dual stable isotope excretion in men consuming a low molybdenum diet.

The aim of this study was to develop a compartmental model of molybdenum metabolism based on stable isotope excretion patterns. Molybdenum (Mo) is an essential trace element in humans, with an estimated safe and adequate daily dietary intake (ESADDI) of 75-250 micrograms Mo/d. Four adult men were fed low molybdenum diets, 22 micrograms Mo/d for a period of 102 d. 97Mo+ and 100Mo stable isotopes, in intravenous and oral doses, respectively, were administered at selected intervals. The resulting 6-d cumulative urinary and fecal isotope excretion data were used to model molybdenum metabolism using SAAM/CONSAM software. A kinetic model, including gastrointestinal (GI), plasma, slow-turnover tissue and fast-turn-over tissue compartments, accurately simulated the observed pattern of urinary and fecal excretion for both stable isotopes in all four subjects. Residence time for molybdenum in the GI tract was estimated at 1.7 +/- 0.4 d. Predicted residence time for plasma molybdenum was 22 +/- 4 min, whereas slow-turnover tissue (possible hepatic) retention averaged 58 +/- 16 d. The model thus permitted estimation of kinetic parameters for molybdenum metabolism in tissues not readily accessible or measurable in humans.

Adult↗
PubMed2007

Molybdenum intake influences molybdenum kinetics in men.

The objectives of this study were to determine physiologic adaptations that occur when humans are exposed to a wide range of molybdenum intake levels and to identify the pathways that are influenced by dietary intake. Four males consumed each of 5 daily molybdenum intakes of 22, 72, 121, 467, and 1490 microg/d (0.23, 0.75, 1.3, 4.9, and 15.5 micromol/d) for 24 d each. During each treatment period, oral and intravenous doses of (100)Mo and (97)Mo were administered. Serial plasma, urine, and fecal samples were analyzed for labeled and unlabeled molybdenum. Compartmental modeling was used to determine rates of distribution and elimination at each dietary intake level. Three pathways were sensitive to daily molybdenum intake. With increasing intake, absorption and urinary molybdenum excretion increased, whereas the fraction deposited in tissues decreased. Kinetic analysis suggested a daily intake of 115-120 microg/d (1.20-1.25 micromol/d) would maintain initial plasma molybdenum levels at their prestudy values and that their prestudy dietary intake was well above the Recommended Dietary Allowance of 45 microg/d. The physiological adaptations to changing intake that the model demonstrated may help prevent molybdenum deficiency and toxicity.

Administration, Oral↗
PubMed1993

Characterization of Rhodobacter capsulatus genes encoding a molybdenum transport system and putative molybdenum-pterin-binding proteins.

The alternative, heterometal-free nitrogenase of Rhodobacter capsulatus is repressed by traces of molybdenum in the medium. Strains carrying mutations located downstream of nifB copy II were able to express the alternative nitrogenase even in the presence of high molybdate concentrations. DNA sequence analysis of a 5.5-kb fragment of this region revealed six open reading frames, designated modABCD, mopA, and mopB. The gene products of modB and modC are homologous to ChlJ and ChlD of Escherichia coli and represent an integral membrane protein and an ATP-binding protein typical of high-affinity transport systems, respectively. ModA and ModD exhibited no homology to known proteins, but a leader peptide characteristic of proteins cleaved during export to the periplasm is present in ModA, indicating that ModA might be a periplasmic molybdate-binding protein. The MopA and MopB proteins showed a high degree of amino acid sequence homology to each other. Both proteins contained a tandem repeat of a domain encompassing 70 amino acid residues, which had significant sequence similarity to low-molecular-weight molybdenum-pterin-binding proteins from Clostridium pasteurianum. Compared with that for the parental nifHDK deletion strain, the molybdenum concentrations necessary to repress the alternative nitrogenase were increased 4-fold in a modD mutant and 500-fold in modA, modB, and modC mutants. No significant inhibition of the heterometal-free nitrogenase by molybdate was observed for mopA mopB double mutants. The uptake of molybdenum by mod and mop mutants was estimated by measuring the activity of the conventional molybdenum-containing nitrogenase. Molybdenum transport was not affected in a mopA mopB double mutant, whereas strains carrying lesions in the binding-protein-dependent transport system were impaired in molybdenum uptake.

Amino Acid Sequence↗
PubMed1988

Stress and activity of molybdenum-containing complex (molybdenum cofactor) in winter wheat seeds.

Molybdenum, applied in vivo, restored the damage from low temperature with winter wheat (Triticum aestivum, var "Sadovo 1") grown on acid soil and, in addition, sharply increased productivity (G Salcheva, D Georgieva, 1982; G Salcheva et al., 1977, 1979). Two fractions with molybdenum-cofactor activity in seeds were detected. One of them has a molecular weight of about 230 kilodaltons corresponding to xanthine oxidase activity and leaf nitrate reductase activity. The other has a molecular weight of about 60 kilodaltons. The ratio between the molybdenum-cofactor activity of these fractions was different in ;mother' seeds used in the experiment, in seeds obtained from the damaged plants, and in seeds obtained from the damaged plants restored by in vivo molybdenum addition. Every one of these fractions consisted of several components in which molybdenum-cofactor activity and stability in vitro was different. We suggest that plants store molybdenum as molybdenum carriers in these low molecular weight fractions.

Journal Article↗