Chemical model systems for cellular nitros(yl)ation reactions

Abstract

S-nitros(yl)ation belongs to the redox-based posttranslational modifications of proteins but the underlying chemistry is controversial. In contrast to current concepts involving the autoxidation of nitric oxide ( NO, nitrogen monoxide), we and others have proposed the formation of peroxynitrite (oxoperoxonitrate (1−)) as an essential intermediate. This requires low cellular fluxes of NO and superoxide ( O 2 −), for which model systems have been introduced. We here propose two new systems for nitros(yl)ation that avoid the shortcomings of previous models. Based on the thermal decomposition of 3-morpholinosydnonimine, equal fluxes of NO and O 2 − were generated and modulated by the addition of NO donors or Cu,Zn-superoxide dismutase. As reactants for S-nitros(yl)ation, NADP +-dependent isocitrate dehydrogenase and glutathione were employed, for which optimal S-nitros(yl)ation was observed at nanomolar fluxes of NO and O 2 − at a ratio of about 3:1. The previously used reactants phenol and diaminonaphthalene (C- and N-nitrosation) demonstrated potential participation of multiple pathways for nitros(yl)ation. According to our data, neither peroxynitrite nor autoxidation of NO was as efficient as the 3 NO/1 O 2 − system in mediating S-nitros(yl)ation. In theory this could lead to an elusive nitrosonium (nitrosyl cation)-like species in the first step and to N 2O 3 in the subsequent reaction. Which of these two species or whether both together will participate in biological S-nitros(yl)ation remains to be elucidated. Finally, we developed several hypothetical scenarios to which the described NO/ O 2 − flux model could apply, providing conditions that allow either direct electrophilic substitution at a thiolate or S-nitros(yl)ation via transnitrosation from S-nitrosoglutathione.