Role of paraquat in the uncoupling of nitric oxide synthase.

Nitric oxide synthase (NOS) oxidizes L-arginine to NO(&z.ccirf;) and L-citrulline. Recent studies have shown that this enzyme can also generate O(2)(&z.ccirf;-) during its enzymatic cycling. Herein, we used spin trapping and electron paramagnetic resonance (EPR) spectroscopy to investigate the impact paraquat has on the transport of electrons through purified neuronal NOS (NOS I). In a concentration-dependent manner, ranging from 10-100 microM of paraquat, paraquat free radical was observed under anaerobic conditions. This demonstrates that NOS shunts electrons to paraquat, thereby uncoupling this enzyme. This resulted in enhanced production of O(2)(&z.ccirf;-) at the expense of NO(&z.ccirf;). Experiments demonstrated that the reductase domain is the site of paraquat-mediated uncoupling of NOS.