Non-enzymatic production of nitric oxide (NO) from NO synthase inhibitors

The gaseous signal molecule, nitric oxide (NO.), is generated enzymatically by NO synthase (NOS) from L-arginine. Overproduction of NO contributes to cell and tissue damage as sequelae of infection and stroke. Strategies to s uppress NO synthesis rely heavily on guanidino-substituted L-arginine analo gs (L-NAME, L-NA, L-NMMA, L-NIO) as competitive inhibitors of NOS, which ar e often used in high doses to compete with millimolar concentrations of int racellular arginine. We show that these analogs are also a source for non-e nzymatically produced NO. Enzyme-independent NO release occurs in the prese nce of NADPH, glutathione, L-cysteine, dithiothreitol and ascorbate, This n on-enzymatic synthesis of NO can produce potentially toxic, micromolar conc entrations of NO and can oppose the effects of NOS inhibition. NO productio n driven by NOS inhibitors was demonstrated ex vivo in the central nervous and peripheral tissues of gastropod molluscs Aplysia and Pleurobranchaea us ing electron paramagnetic resonance and spin-trapping techniques. These res ults have important implications for therapeutic regulation of NO homeostas is. (C) 1998 Academic Press.