title: Mechanistic studies of the reactions of the reduced vitamin B12 derivatives with the HNO donor Piloty's acid: further evidence for oxidation of cob(I)alamin by (H)NO.
abstract: There is accumulating evidence for the existence of HNO in biological systems. Compared with NO (˙NO), much less is known about the chemical and biochemical reactivity of HNO. Kinetic and mechanistic studies have been carried out on the reaction between the vitamin B12-derived radical complex cob(II)alamin (Cbl(II)˙, Cbl(II)) with the widely used HNO donor Piloty's acid (PA). A stoichiometry of 1 : 2 Cbl(II) : PA was obtained and PA decomposition to HNO and benzenesulfinate (C6H5SO2(-)) is the rate-determining step. No evidence was found for nitrite (Griess assay), ammonia (Nessler's test) or NH2OH (indooxine test) in the product solution, and it is likely that HNO is instead reduced to N2. A mechanism is proposed in which reduction of Cbl(II) by (H)NO results in formation of cob(I)alamin (Cbl(I)(-)) and ˙NO. The Cbl(I)(-) intermediate is subsequently oxidized back to Cbl(II) by a second (H)NO molecule, and Cbl(II) reacts rapidly with ˙NO to form nitroxylcobalamin (NOCbl). Separate studies on the reaction between Cbl(I)(-) and PA shows that this system involves an additional step in which Cbl(I)(-) is first oxidized by (H)NO to Cbl(II), which reacts further with (H)NO to form NOCbl, with an overall stoichiometry of 1 : 3 Cbl(I)(-) : PA. Experiments in the presence of nitrite for both systems support the involvement of a Cbl(I)(-) intermediate in the Cbl(II)/PA reaction. These systems provide the second example of oxidation of cob(I)alamin by (H)NO.
authors.affiliation: Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, USA and Division of Science, Mathematics, and Physical Education, Western Nebraska Community College, Scottsbluff, Nebraska 69361, USA.
authors.forename: Harishchandra
authors.initials: H
authors.lastname: Subedi
authors.affiliation: School of Applied Sciences, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand. nbrasch@aut.ac.nz.
authors.forename: Nicola E
authors.initials: NE
authors.lastname: Brasch
mesh_terms: D006877:Hydroxamic Acids / Q000737:chemistry*; D009589:Nitrogen Oxides / Q000737:chemistry*; D010084:Oxidation-Reduction; D013449:Sulfonamides / Q000737:chemistry*; D014805:Vitamin B 12 / Q000737:chemistry*
chemical_list: D006877:Hydroxamic Acids; D009589:Nitrogen Oxides; D013449:Sulfonamides; C097220:Piloty's acid; C039900:nitroxyl; D014805:Vitamin B 12