Nitrite reduction by xanthine oxidase family enzymes: a new class of nitrite reductases

Abstract

Mammalian xanthine oxidase (XO) and Desulfovibrio gigas aldehyde oxidoreductase (AOR) are members of the XO family of mononuclear molybdoenzymes that catalyse the oxidative hydroxylation of a wide range of aldehydes and heterocyclic compounds. Much less known is the XO ability to catalyse the nitrite reduction to nitric oxide radical (NO). To assess the competence of other XO family enzymes to catalyse the nitrite reduction and to shed some light onto the molecular mechanism of this reaction, we characterised the anaerobic XO-and AORcatalysed nitrite reduction. The identification of NO as the reaction product was done with a NO-selective electrode and by electron paramagnetic resonance (EPR) spectroscopy. The steady-state kinetic characterisation corroborated the XO-catalysed nitrite reduction and demonstrated, for the first time, that the prokaryotic AOR does catalyse the nitrite reduction to NO, in the presence of any electron donor to the enzyme, substrate (aldehyde) or not (dithionite). Nitrite binding and reduction was shown by EPR spectroscopy to occur on a reduced molybdenum centre. A molecular mechanism of AOR-and XO-catalysed nitrite reduction is discussed, in which the higher oxidation states of molybdenum seem to be involved in oxygen-atom insertion, whereas the lower oxidation states would favour oxygenatom abstraction. Our results define a new catalytic performance for AOR-the nitrite reduction-and propose a new class of molybdenum-containing nitrite reductases.

Nitrite Reductase and Nitric-oxide Synthase Activity of the Mitochondrial Molybdopterin Enzymes mARC1 and mARC2

“…In xanthine oxidase and aldehyde oxidase, the molybdopterin active site is the location of nitrite reduction to NO. Presumably, the oxygen atom of the nitrite anion is transferred to molybdenum via an oxygen atom transfer reaction, releasing NO (49). The molybdopterin active site of mARC is similar to xanthine oxidase with one central difference, xanthine oxidase and aldehyde oxidase contain a sulfido ligand, whereas mARC and sulfite oxidase use as the ligand the sulfur atom of a cysteine side-chain, Cys-273 mARC-1 and Cys272 mARC-2, which is strictly conserved in eukaryotic and prokaryotic mARC homologues (29).…”

Nitric Oxide–Triggered Remodeling of Chloroplast Bioenergetics and Thylakoid Proteins upon Nitrogen Starvation inChlamydomonas reinhardtii

“…For instance, in many organisms, including vascular plants, xanthine oxidasedehydrogenase (XOR), aldehyde oxidase (AO), and sulfite oxidase (SO), all MoCo-requiring enzymes, can reduce nitrite to NO (Tewari et al, 2009;B.L. Wang et al, 2010;Maia and Moura, 2011), even under normoxia, when using NADH as a substrate (Li et al, 2004). In that respect, we noted that the induction of LAO1, which displays the same regulation pattern as the degradation of cytochrome b 6 f and Rubisco, as it is much delayed and limited in genetic or environmental contexts that prevent the degradation of these proteins, was even more delayed in strain nit4-104 lacking the four enzymes NaR, AO, XOR, and SO, because of the absence of MoCo, than in strain nit1-137 lacking NaR only.…”

Modulating endothelial nitric oxide synthase: a new cardiovascular therapeutic strategy

“…The precise mechanism of organic nitrate bioactivation is incompletely understood but is thought to involve both enzymatic and nonenzymatic mechanisms. The active metabolites following enzymatic reduction by glutathione S-transferases (69), cytochrome P-450 reductases (1), xanthine oxidoreductases (XORs) (79), and mitochondrial aldehyde dehydrogenases (ALDH-2) (23), in turn, activate the intracellular NO receptor enzyme soluble guanylate cyclase in the vessel wall. Activated soluble guanylate cyclase increases tissue levels of the second-messenger cGMP, which ultimately mediates vasorelaxation via a cGMP-dependent protein kinase.…”