Analysis of a ferric leghemoglobin reductase from cowpea (Vigna unguiculata) root nodules

Multiple putative methemoglobin reductases in C. reinhardtii may support enzymatic functions for its multiple hemoglobins

The ubiquitous nature of hemoglobins, their presence in multiple forms and low cellular expression in organisms suggests alternative physiological functions of hemoglobins in addition to oxygen transport and storage. Previous research has proposed enzymatic function of hemoglobins such as nitric oxide dioxygenase, nitrite reductase and hydroxylamine reductase. In all these enzymatic functions, active ferrous form of hemoglobin is converted to ferric form and reconversion of ferric to ferrous through reduction partners is under active investigation. The model alga C. reinhardtii contains multiple globins and is thus expected to have multiple putative methemoglobin reductases to augment the physiological functions of the novel hemoglobins. In this regard, three putative methemoglobin reductases and three algal hemoglobins were characterized. Our results signify that the identified putative methemoglobin reductases can reduce algal methemoglobins in a nonspecific manner under in vitro conditions. Enzyme kinetics of two putative methemoglobin reductases with methemoglobins as substrates and in silico analysis support interaction between the hemoglobins and the two reduction partners as also observed in vitro. Our investigation on algal methemoglobin reductases underpins the valuable chemistry of nitric oxide with the newly discovered hemoglobins to ensure their physiological relevance, with multiple hemoglobins probably necessitating the presence of multiple reductases.

A self-induction method to produce high quantities of recombinant functional flavo-leghemoglobin reductase

Ferric leghemoglobin reductase (FLbR) is able to reduce ferric leghemoglobin (Lb3+) to ferrous (Lb2+) form. This reaction makes Lb functional in performing its role since only reduced hemoglobins bind O2. FLbR contains FAD as prosthetic group to perform its activity. FLbR-1 and FLbR-2 were isolated from soybean root nodules and it has been postulated that they reduce Lb3+. The existence of Lb2+ is essential for the nitrogen fixation process that occurs in legume nodules; thus, the isolation of FLbR for the study of this enzyme in the nodule physiology is of interest. However, previous methods for the production of recombinant FLbR are inefficient as yields are too low. We describe the production of a recombinant FLbR-2 from Escherichia coli BL21(DE3) by using an overexpression method based on the self-induction of the recombinant E. coli. This expression system is four times more efficient than the previous overexpression method. The quality of recombinant FLbR-2 (based on spectroscopy, SDS-PAGE, IEF, and native PAGE) is comparable to that of the previous expression system. Also, FLbR-2 is purified near to homogeneity in only few steps (in a time scale, the full process takes 3 days). The purification method involves affinity chromatography using a Ni-nitrilotriacetic acid column. Resulting rFLbR-2 showed an intense yellow color, and spectral characterization of rFLbR-2 indicated that rFLbR-2 contains flavin. Pure rFLbR-2 was incubated with soybean Lba and NADH, and time drive rates showed that rFLbR-2 efficiently reduces Lb3+.

Functional characterization and expression of a cytosolic iron-superoxide dismutase from cowpea root nodules

An iron-superoxide dismutase (FeSOD) with an unusual subcellular localization, VuFeSOD, has been purified from cowpea (Vigna unguiculata) nodules and leaves. The enzyme has two identical subunits of 27 kD that are not covalently bound. Comparison of its N-terminal sequence (NVAGINLL) with the cDNA-derived amino acid sequence showed that VuFeSOD is synthesized as a precursor with seven additional amino acids. The mature protein was overexpressed in Escherichia coli, and the recombinant enzyme was used to generate a polyclonal monospecific antibody. Phylogenetic and immunological data demonstrate that there are at least two types of FeSODs in plants. An enzyme homologous to VuFeSOD is present in soybean (Glycine max) and common bean (Phaseolus vulgaris) nodules but not in alfalfa (Medicago sativa) and pea (Pisum sativum) nodules. The latter two species also contain FeSODs in the leaves and nodules, but the enzymes are presumably localized to the chloroplasts and plastids. In contrast, immunoblots of the soluble nodule fraction and immunoelectron microscopy of cryo-processed nodule sections demonstrate that VuFeSOD is localized to the cytosol. Immunoblot analysis showed that the content of VuFeSOD protein increases in senescent nodules with active leghemoglobin degradation, suggesting a direct or indirect (free radical-mediated) role of the released Fe in enzyme induction. Therefore, contrary to the widely held view, FeSODs in plants are not restricted to the chloroplasts and may become an important defensive mechanism against the oxidative stress associated with senescence.

Rapid spectrophotometric determination of 2,4,6-trinitrotoluene in a Pseudomonas enzyme assay

Although TNT (2,4,6-trinitrotoluene) and its degradation products can be quantified by HPLC, this method is not suitable for simultaneous analyses of the numerous samples typically encountered in enzyme studies. To solve this problem, we developed a simple and rapid spectrophotometric assay for TNT and tested the procedure using partially purified nitroreductase(s) from a Pseudomonas aeruginosa isolate, which transformed TNT in the culture medium. In highly alkaline solution, TNT (pK(a)=11.99) exhibits significant absorbance at 447 nm, while major metabolites, 2-amino-4, 6-dinitrotoluene (2ADNT), 4-amino-2,6-dinitrotoluene (4ADNT), and 2,6-diamino-4-nitrotoluene (2,6DANT) display no absorbance at this wavelength. Assay mixtures of TNT, Tris-HCl buffer, a reductant, and the enzyme(s) were analyzed by measuring absorbance 4 min after adjusting the pH to 12.2. TNT transformation to colorless metabolites was linear with respect to protein and substrate concentrations. Using the assay, we determined that TNT nitroreductase(s) from the isolate required an electron donor and preferred NADH to NADPH. TNT transformation increased when NAD was recycled to NADH using glucose-6-phosphate (GP) and glucose-6-phosphate dehydrogenase (GPDH). Enzymatic transformation of TNT was completely inhibited by Cu(2+) (5 mM) and was partially inhibited by other divalent metallic cations. Because the assay is sensitive to ammonium sulfate, dithiothreitol, ascorbic acid, and sodium phosphate, extracts should be assayed in the absence of these components.