Flavin-mediated reduction of ferric leghemoglobin from soybean nodules

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Abstract
The reduction of ferric leghemoglobin (Lb3+) from soybean (Glycine max (L.) Merr.) nodules by riboflavin, FMN and FAD in the presence of NAD(P)H was studied in vitro. The system NAD(P)H + flavin reduced Lb3+ to oxyferrous (Lb2+ · O2) or deoxyferrous (Lb2+) leghemoglobin in aerobic or anaerobic conditions, respectively. In the absence of O2 the reaction was faster and more effective (i.e. less NAD(P)H oxidized per mole Lb3+ reduced) than in the presence of O2; this phenomenon was probably because O2 competes with Lb3+ for reductant, thus generating activated O2 species. The flavin-mediated reduction of Lb3+ did not entail production of superoxide or peroxide, indicating that NAD(P)H-reduced flavins were able to reduce Lb3+ directly. The NAD(P)H + flavin system also reduced the complexes Lb3+ · nicotinate and Lb3+ · acetate to Lb2+ · O2, Lb2+ or Lb2+ · nicotinate, depending on the concentrations of ligands and of O2. In the presence of 200 μM nitrite most Lb remained as Lb3+ in aerobic conditions but the nitrosyl complex (Lb2+ · NO) was generated in anaerobic conditions. The above-mentioned characteristics of the NAD(P)H + flavin system, coupled with its effectiveness in reducing Lb3+ at physiological levels of NAD(P)H and flavins in soybean nodules, indicate that this mechanism may be especially important for reducing Lb3+ in vivo.