Nitrosation of melatonin by nitric oxide and peroxynitrite

Peroxynitrite (ONOO-) is an endogenous molecule, formed by rapid coupling b etween (NO)-N-. and O-2(.-), ONOO- is known to be a strong oxidant of thiol s and metalloorganic compounds and also a nitrating agent of aromatic compo unds such as tyrosine. However, its chemistry is not yet well elucidated un der physiological conditions. Melatonin, which is an indole-amine produced by the pineal gland and other organs, has antioxidant properties. We show t hat melatonin reacts with ONOO- in phosphate-buffered solutions. We provide evidence of nitrosation and oxidation at the pyrrole nitrogen leading to 1 -nitrosomelatonin and 1-hydroxymelatonin, these being the major reactions i n aqueous phosphate-buffered solutions besides other aromatic hydroxylation s and nitration. 4-Nitromelatonin is formed, but in small amounts. The kine tics of all transformations were strictly dependent on ONOO- decay, whereas yields varied with pH and the presence of CO2. The N-oxidation became comp etitive with nitrosation at pH 7.4, in medium containing a sufficient amoun t of CO2. A proposed mechanism involves the transient formation of melatony l radical and ONOO. radical derived from ONOO- decay.