Endogenous dimethylarginine as an inhibitor of nitric oxide synthesis.

Nitric oxide (NO) is a widespread biological mediator with myriad functions. We have demonstrated that methylated arginines capable of inhibiting NO synthesis circulate in the plasma of healthy volunteers and are excreted unchanged in the urine. Up to 10 mg of asymmetric dimethylarginine is excreted in the urine every day, and this compound inhibits NO synthesis in vitro and in vivo, in animals and in humans. This finding raises the possibility that these compounds may act as endogenous regulators of the L-arginine:NO pathway in health and disease.