title: Thiols enhance NO formation from nitrate photolysis.
abstract: Nitrate is generally considered an inert oxidative breakdown product of nitric oxide (NO). Whereas it has been shown that limited amounts of NO are produced during the photolysis of nitrate in aqueous solution, the photochemistry of nitrate in biological matrices such as plasma is unknown. We hypothesized that thiols, which are ubiquitously present in biological systems, may significantly enhance NO-quantum yields from nitrate photolysis. Exposure of fresh human plasma to high-intensity UV-light resulted in NO-formation (19 +/- 3 nmol/l/min) as measured by gas phase chemiluminescence, and this signal was almost completely abolished by the removal of plasma N-oxides (2 +/- 1 nmol/l/min). Reconstitution of NOx-depleted plasma samples with a physiological concentration of nitrate, but not nitrite, restored photolytic NO-generation to values comparable to naïve plasma. Addition of the thiol-reducing agent, dithiothreitol or the sulfhydryl-bearing amino acid, L-cysteine increased NO-formation above control levels. Thiol-blockade by either N-ethylmaleimide (NEM) or mercuric chloride (HgCl2) reduced basal NO formation from 19 +/- 3 to 7 +/- 2 and 4 +/- 1 nmol/l/min, respectively. Exposure of plasma to UV-light increased NO-adduct concentrations from 18 +/- 5 to 1662 +/- 658 nmol/l. Collectively, our results show that thiols facilitate photolytic conversion of nitrate to NO and NO-adducts such as S-nitrosothiols. This may lead to substantial overestimation of the latter when photolysis-based methodologies are used for their determination. Whether this novel reaction channel also has in vivo relevance remains to be investigated.
authors.affiliation: Department of Medicine, Division of Cardiology, Pulmonary Diseases, Angiology, Heinrich-Heine-University, Duesseldorf, Germany.
authors.forename: André
authors.initials: A
authors.lastname: Dejam
authors.forename: Petra
authors.initials: P
authors.lastname: Kleinbongard
authors.forename: Tienush
authors.initials: T
authors.lastname: Rassaf
authors.forename: Sandra
authors.initials: S
authors.lastname: Hamada
authors.forename: Putrika
authors.initials: P
authors.lastname: Gharini
authors.forename: Juan
authors.initials: J
authors.lastname: Rodriguez
authors.forename: Martin
authors.initials: M
authors.lastname: Feelisch
authors.forename: Malte
authors.initials: M
authors.lastname: Kelm
mesh_terms: D009566:Nitrates / Q000737:chemistry*; D009569:Nitric Oxide / Q000138:chemical synthesis*; D010782:Photolysis; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D009566:Nitrates; D013438:Sulfhydryl Compounds; D009569:Nitric Oxide