Figure 1: L-arginine, the common substrate to both the nitric oxide synthase and the arginase pathway. L-arginine is catalyzed by both arginase and nitric oxide. Arginase produces L-ornithine and urea that can act in various organs. In contrast, nitric oxide can be produced by both constitutive (cNOS) and inducible (iNOS) nitric oxide synthases and have pathophysiological roles important in health and diseases via the direct or indirect effects on oxidative stress production. Thus, arginase regulates the production of NO, and NO regulates the activity of arginase by substrate competition.