title: Reduction of nitrite to nitric oxide catalyzed by xanthine oxidoreductase.
abstract: Xanthine oxidase (XO) was shown to catalyze the reduction of nitrite to nitric oxide (NO), under anaerobic conditions, in the presence of either NADH or xanthine as reducing substrate. NO production was directly demonstrated by ozone chemiluminescence and showed stoichiometry of approximately 2:1 versus NADH depletion. With xanthine as reducing substrate, the kinetics of NO production were complicated by enzyme inactivation, resulting from NO-induced conversion of XO to its relatively inactive desulfo-form. Steady-state kinetic parameters were determined spectrophotometrically for urate production and NADH oxidation catalyzed by XO and xanthine dehydrogenase in the presence of nitrite under anaerobic conditions. pH optima for anaerobic NO production catalyzed by XO in the presence of nitrite were 7.0 for NADH and </=6.0 for xanthine. Involvement of the molybdenum site of XO in nitrite reduction was shown by the fact that alloxanthine inhibits xanthine oxidation competitively with nitrite. Strong preference for Mo=S over Mo=O was shown by the relatively very low NADH-nitrite reductase activity shown by desulfo-enzyme. The FAD site of XO was shown not to influence nitrite reduction in the presence of xanthine, although it was clearly involved when NADH was the reducing substrate. Apparent production of NO decreased with increasing oxygen tensions, consistent with reaction of NO with XO-generated superoxide. It is proposed that XO-derived NO fulfills a bactericidal role in the digestive tract.
authors.affiliation: Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom.
authors.forename: B L
authors.initials: BL
authors.lastname: Godber
authors.forename: J J
authors.initials: JJ
authors.lastname: Doel
authors.forename: G P
authors.initials: GP
authors.lastname: Sapkota
authors.forename: D R
authors.initials: DR
authors.lastname: Blake
authors.forename: C R
authors.initials: CR
authors.lastname: Stevens
authors.forename: R
authors.initials: R
authors.lastname: Eisenthal
authors.forename: R
authors.initials: R
authors.lastname: Harrison
mesh_terms: D000693:Anaerobiosis; D000818:Animals; D001713:Biphenyl Compounds / Q000494:pharmacology; D020134:Catalytic Domain; D002417:Cattle; D005182:Flavin-Adenine Dinucleotide / Q000378:metabolism; D005420:Flavoproteins / Q000378:metabolism; D007700:Kinetics; D008667:Metalloproteins / Q000378:metabolism; D008892:Milk / Q000201:enzymology; D008982:Molybdenum / Q000378:metabolism; D009243:NAD / Q000378:metabolism; D009569:Nitric Oxide / Q000378:metabolism*; D009573:Nitrites / Q000378:metabolism*; D009861:Onium Compounds / Q000494:pharmacology; D010084:Oxidation-Reduction; D019820:Xanthine / Q000378:metabolism; D014968:Xanthine Dehydrogenase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D014969:Xanthine Oxidase / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D001713:Biphenyl Compounds; D005420:Flavoproteins; D008667:Metalloproteins; D009573:Nitrites; D009861:Onium Compounds; D009243:NAD; C031291:diphenyliodonium; D005182:Flavin-Adenine Dinucleotide; D019820:Xanthine; D009569:Nitric Oxide; D008982:Molybdenum; D014968:Xanthine Dehydrogenase; D014969:Xanthine Oxidase