Regulation of nitric oxide synthase]

Nitric oxide (NO) synthase is known to be widely distributed in various cells. We characterized this enzyme using partially purified enzyme fractions from rat neutrophils, macrophages, and cerebellum. The cerebellar fraction required Ca(2+)-calmodulin, while that from macrophages required neither calmodulin nor Ca2+. The neutrophil fraction required Ca2+. The enzyme was inhibited by analogues of the substrate, L-arginine. N omega-nitro-L-arginine (NNA) was 20 times more potent than L-NG-monomethyl-arginine (NMA) in blocking the cerebellar enzyme. In contrast, NNA and NMA were about equipotent against neutrophil and macrophage enzymes. These data suggest that the enzyme is regulated by 3 different mechanisms in these cells, with differences between the cerebellar and neutrophil or macrophage enzyme in the catalytic binding site.