Electron-paramagnetic-resonance studies of leghaemoglobins from soya-bean and cowpea root nodules. Identification of nitrosyl-leghaemoglobin in crude leghaemoglobin preparations

Abstract

1. Leghaemoglobins from soya-bean (Glycine max) and cowpea (Vigna unguiculata) root nodules were purified by chromatography on DEAE-cellulose phosphate columns at pH8.0 and pH5.8, to avoid the relatively low pH (5.2) commonly used to purify these proteins. 2. E.p.r. (electron-paramagnetic-resonance) spectra of the fluoride, azide, hydroxide and cyanide complexes of these ferric leghaemoglobins were very similar to the spectra of the corresponding myoglobin derivatives, indicating that the immediate environment of the iron in leghaemoglobin and myoglobin is similar, an imidazole moiety of histidine being the proximal ligand to the haem iron [cf. Appleby, Blumberg, Peisach, Wittenberg & Wittenberg (1976) J. Biol. Chem.251, 6090–6096]. 3. E.p.r. spectra of the acid-metleghaemoglobins showed prominent high-spin features very near g=6 and g=2 and, unlike myoglobin, small low-spin absorptions near g=2.26, 2.72 and 3.14. The width of the g=6 absorption derivative at 10–20K was about 4–4.5mT, similar to the value for acid-methaemoglobin. In contrast, a recently published (Appleby et al., 1976) spectrum of acid-metleghaemoglobin a had less high-spin character and a much broader absorption derivative around g=6. 4. E.p.r. spectra of ferric leghaemoglobin nicotinate and imidazole complexes suggest that the low-spin absorption near g=3.14 can be attributed to a trace of ferric leghaemoglobin nicotinate, and those near g=2.26 and 2.72 are from an endogenous dihistidyl haemichrome. 5. A large e.p.r. signal at g=2 in all samples of crude leghaemoglobin was shown to be from nitrosyl-leghaemoglobin. A soya-bean sample contained 27±3% of the latter. A previously unidentified form of soya-bean ferrous leghaemoglobin a was shown to be its nitrosyl derivative. If this is not an artifact, and occurs in the root nodule, the nitrosyl radical may interfere with the function of leghaemoglobin.