Regulation of nitrogenase activity in aerobes by Mg2+ availability: an hypothesis

Determination of nucleoside triphosphatase activities from measurement of true inorganic phosphate in the presence of labile phosphate compounds

One of the most common assays for nucleoside triphosphatase (NTPase) activity entails the quantification of inorganic phosphate (P_i) as a colored phosphomolybdate complex at low pH. While this assay is very sensitive, it is not selective for P_i in the presence of labile organic phosphate compounds (OPCs). Since NTPase activity assays typically require a large excess of OPCs, such as nucleotides, selectivity for P_i in the presence of OPCs is often critical in evaluating enzyme activity. Here we present an improved method for the measurement of enzymatic nucleotide hydrolysis as P_i released, which achieves selectivity for P_i in the presence of OPCs while also avoiding the costs and hazards inherent in other methods for measuring nucleotide hydrolysis. We apply this method to the measurement of ATP hydrolysis by nitrogenase and GTP hydrolysis by elongation factor G (EF-G) in order to demonstrate the broad applicability of our method for the determination of nucleotide hydrolysis in the presence of interfering OPCs.

Ammonium inhibition of nitrogenase activity in Herbaspirillum seropedicae

The effect of oxygen, ammonium ion, and amino acids on nitrogenase activity in the root-associated N2-fixing bacterium Herbaspirillum seropedicae was investigated in comparison with Azospirillum spp. and Rhodospirillum rubrum. H. seropedicae is microaerophilic, and its optimal dissolved oxygen level is from 0.04 to 0.2 kPa for dinitrogen fixation but higher when it is supplied with fixed nitrogen. No nitrogenase activity was detected when the dissolved O2 level corresponded to 4.0 kPa. Ammonium, a product of the nitrogenase reaction, reversibly inhibited nitrogenase activity when added to derepressed cell cultures. However, the inhibition of nitrogenase activity was only partial even with concentrations of ammonium chloride as high as 20 mM. Amides such as glutamine and asparagine partially inhibited nitrogenase activity, but glutamate did not. Nitrogenase in crude extracts prepared from ammonium-inhibited cells showed activity as high as in extracts from N2-fixing cells. The pattern of the dinitrogenase and the dinitrogenase reductase revealed by the immunoblotting technique did not change upon ammonium chloride treatment of cells in vivo. No homologous sequences were detected with the draT-draG probe from Azospirillum lipoferum. There is no clear evidence that ADP-ribosylation of the dinitrogenase reductase is involved in the ammonium inhibition of H. seropedicae. The uncoupler carbonyl cyanide m-chlorophenylhydrazone decreased the intracellular ATP concentration and inhibited the nitrogenase activity of whole cells. The ATP pool was not significantly disturbed when cultures were treated with ammonium in vivo. Possible mechanisms for inhibition by ammonium of whole-cell nitrogenase activity in H. seropedicae are discussed.

Adenine nucleotide levels in Rhodospirillum rubrum during switch-off of whole-cell nitrogenase activity

Adenine nucleotide pools were measured in Rhodospirillum rubrum cultures that contained nitrogenase. The average energy charge [([ATP] + 1/2[ADP])/([ATP] + [ADP] + [AMP])] was found to be 0.66 and 0.62 in glutamate-grown and N-limited cultures respectively. Treatment of glutamate-grown cells with darkness, ammonia, glutamine, carbonyl cyanide m-chlorophenylhydrazone, or phenazine methosulphate resulted in perturbations in the adenine nucleotide pools, and led to loss of whole-cell nitrogenase activity and modification in vivo of the Fe protein. Treatment of N-limited cells resulted in similar changes in adenine nucleotide pools but not enzyme modification. No correlations were found between changes in adenine nucleotide pools or ratios of these pools and switch-off of nitrogenase activity by Fe protein modification in vivo. Phenazine methosulphate inhibited whole-cell activity at low concentrations. The effect on nitrogenase activity was apparently independent of Fe protein modification.

Chemically coupled spectrophotometric assays based on flow injection analysis: determination of nitrogenase by assays for creatine, ammonia, hydrazine, phosphate, and dithionite

Micromethods of direct chemical coupling have been developed for several different enzyme reactions, using the principles of flow injection analysis. Samples of 1-25 microliters are injected into a flowing stream of color-forming reagents and the peak of color change is measured after about 1 min. Alternatively, continuous slow infusion of a reacting system (5-100 microliters/min) gives a continuous change of color which can be monitored to derive enzyme reaction rates. These techniques are highly sensitive, requiring a few nanomoles of the substance being detected. Phosphate, ammonia, dithionite, creatine, and hydrazine have been measured. Consumption of reagents is less than 75 ml per hour; typical sample throughout is 30-40 samples per hour by the injection method, and 5 samples per hour by continuous infusion. The procedure has been applied to nitrogenase, continuously monitoring creatine produced from creatine phosphate by creatine kinase which is used to supply a constant level of ATP for nitrogenase. In this way nitrogenase activity can be determined over a wide range of enzyme concentrations. Production of inorganic phosphate directly from ATP, by injection of formaldehyde-quenched samples, was used when coupling to creatine kinase was not possible. Both injection of aliquots and continuous infusion were used for detection of hydrazine during nitrogenase reduction of azide, and the injection method has been used for ammonia assay during dinitrogen reduction. Dithionite oxidation was measured directly from decolorization of iodine, after trapping both dithionite and bisulfite with formaldehyde.

Interaction of nitrogenase with nucleotide analogs of ATP and ADP and the effect of metal ions on ADP inhibition

The interaction of a large number of ATP and ADP analogs with nitrogenase from Azotobacter vinelandii, Klebsiella pneumoniae, and Clostridium pasteurianum has been examined. Only 1,N6-etheno-ATP and 2'-deoxy-ATP served as substrates for acetylene reduction. Other triphosphates including GTP, ITP, 8-Br-ATP, alpha,beta-methylene ATP, beta,gamma-methylene ATP, 6-chloropurine riboside triphosphate, and AMP-PNP were inert, showing less than 50% inhibition at levels up to two- to fivefold greater than ATP. Xanthosine triphosphate behaved simply as a chelator of magnesium, activating the enzyme at low levels but strongly inhibiting at high levels. When nucleotide diphosphates were tested as inhibitors with enzyme from A. vinelandii, GDP, dGDP, and 6-chloropurine riboside diphosphate were ineffective, XDP was three- to fivefold less effective, and dADP and 1,N6-etheno-ADP were about equally as effective as ADP. With enzyme from C. pasteurianum, dADP was twofold less effective than ADP, XDP was fivefold less effective, and IDP and 1,N6-etheno-ADP appeared to be ineffective. Results with enzyme from K. pneumoniae were very similar to those obtained with A. vinelandii. Different metal ions were tested in the presence of both ATP and ADP to determine whether preferential binding to one nucleotide or the other might alter the ADP/ATP ratio needed for 50% inhibition of activity. Magnesium and manganese gave the same ratio, while with Fe and Co, slightly less ADP was required for equivalent inhibition. Nickel appeared to reduce the sensitivity of A. vinelandii nitrogenase to ADP inhibition while increasing that of C. pasteurianum, but both effects were less than twofold. Calcium, strontium, and aluminum ions were inert with enzymes from these organisms. Cd and Zn were also ineffective with K. pneumoniae. Two isomers of ATP beta S were prepared by enzymatic synthesis from ADP beta S. The A form was a more potent inhibitor of A. vinelandii nitrogenase.

Adenine nucleotide levels in and nitrogen fixation by the cyanobacterium Anabaena sp. strain 7120

Adenine nucleotide levels were determined in whole filaments of Anabaena sp. 7120 grown under different N2-fixing or non-N2-fixing conditions. These were compared with levels in isolated heterocysts, Rhodospirillum rubrum, and Azotobacter vinelandii. Adenine nucleotides in whole filaments of Anabaena sp. do not reflect the energetic expense of N2 fixation as they do in R. rubrum and A. vinelandii. However, adenine nucleotide levels in heterocysts were similar to the levels found in N2-fixing R. rubrum, i.e., an ATP:ADP ratio near 1 and an energy charge between 0.5 and 0.7. Nitrogenase activity was only 50% of optimal in permeabilized heterocysts at an exogenous ATP:ADP ratio of 3.33. Hydrogen, which increases acetylene reduction activity, also causes a transient increase (2 to 5 min) in the ATP:ADP ratio. Hydrogen has little effect on energy charge.

Nitrogenase activity and membrane electrogenesis in the cyanobacterium Plectonema boryanum

The relationships between nitrogenase activity (acetylene reduction) and the transplasmalemma proton electrochemical potential gradient (delta muH+) have been investigated using the non-heterocystous filamentous cyanobacterium Plectonema boryanum. By selectively modifying the chemical (delta pH) and electrical (delta psi) components of delta muH+ under conditions in which the size of the ATP pool remained unaffected, nitrogenase activity was found to be dependent on, or regulated by delta psi. When the ATP pool decreased, concomitant with a decreased internal pH (pHi) the requirement for delta psi was modified. The observed reduction in the intracellular ATP pool and the decreased ATP:ADP ratio also correlated with an inhibition of nitrogenase activity. The data are consistent with a model in vivo in which reductant supply to nitrogenase is regulated by, or dependent on an energised plasmalemma and where there is a fine balance between the supply of reductant and ATP for nitrogenase activity. The correlation observed between delta psi and nitrogenase activity extends our previous observations using the heterocystous cyanobacterium Anabaena variabilis.