Characterization of a heme c nitrite reductase from a non-ammonifying microorganism, Desulfovibrio vulgaris Hildenborough

A cytochrome c nitrite reductase (NiR) was purified for the first time from a microorganism not capable of growing on nitrate, the sulfate-reducing ba cterium Desulfovibrio vulgaris Hildenborough. It was isolated from the memb ranes as a large heterooligomeric complex of 760 kDa, containing two cytoch rome c subunits of 56 and 18 kDa. This complex has nitrite and sulfite redu ctase activities of 685 mu mol NH4+/min/mg and 1.0 mu mol H-2/min/mg. The e nzyme was studied by UV-visible and electron paramagnetic resonance (EPR) s pectroscopies. The overall redox behavior was determined through a visible redox titration. The data were analyzed with a set of four redox transition s, with an E-0' of +160 mV (12% of total absorption), -5 mV (38% of total a bsorption), -110 mV (38% of total absorption) and -210 mV (12% of total abs orption) at pH 7.6. The EPR spectra of oxidized and partially reduced NIR s how a complex pattern, indicative of multiple heme-heme magnetic interactio ns. It was found that D. vulgaris Hildenborough is not capable of using nit rite as a terminal electron acceptor. These results indicate that in this o rganism the NiR is not involved in the dissimilative reduction of nitrite, as is the case with the other similar enzymes isolated so far. The possible role of this enzyme in the detoxification of nitrite and/or in the reducti on of sulfite is discussed. (C) 2000 Elsevier Science B.V. All rights reser ved.