CONICET | Buscador de Institutos y Recursos Humanos

A new type of physiologically relevant nitrosamines have been recently recognized, the N1-nitrosoindoles. The possible pathways by which N1-nitrosomelatonin (NOMel) can react in physiological environments have been studied. Our results show that NOMel slowly decomposes spontaneously in aqueous solution, generating melatonin as the main organic product (k ) (3.7 (1.1) 10-5 s-1, Tris-HCl (0.2 M) buffer, pH 7.4 at 37 °C, anaerobic). This rate is accelerated by acidification (kpH 5.8 ) (4.5 ( 0.7) 10-4 s-1, kpH 8.8 ) (3.9 ( 0.6) 10-6 s-1, Tris-HCl (0.2 M) buffer at 37 °C), by the presence of O2 (ko ) (9.8 ( 0.1) 10-5 s-1, pH 7.4, 37 °C, [NOMel] ) 0.1 mM, P(O2) ) 1 atm), and by the presence of the spin trap TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl; ko ) (2.0 ( 0.1) 10-4 s-1, pH 7.4, 37 °C, [NOMel] ) 0.1 mM, [TEMPO] ) 9 mM). We also found that NOMel can transnitrosate to L-cysteinate, producing S-nitrosocysteine and melatonin (k ) 0.127 ( 0.002 M-1 s-1, Tris-HCl (0.2 M) buffer, pH 7.4 at 37 °C). The reaction of NOMel with ascorbic acid as a reducing agent has also been studied. This rapid reaction produces nitric oxide and melatonin. The saturation of the observed rate constant (k ) (1.08 ( 0.04) 10-3 s-1, Tris-HCl (0.2 M) buffer, pH 7.4 at 37 °C) at high ascorbic acid concentration (100-fold with respect to NOMel) and the pH independence of this reaction in the pH range 7-9 indicate that the reactive species are ascorbate and melatonyl radical originated from the reversible homolysis of NOMel. Taking into account kinetic and DFT calculation data, a comprehensive mechanism for the denitrosation of NOMel is proposed. On the basis of our kinetics results, we conclude that under physiological conditions NOMel mainly reacts with endogenous reducing agents (such as ascorbic acid), producing nitric oxide and melatonin.

enviar mensaje