Dioxygen-dependent metabolism of nitric oxide in mammalian cells

Steady-state gradients of NO within tissues and cells are controlled by rates of NO synthesis, diffusion, and decomposition. Mammalian cells and tissues actively decompose NO. Of several cell lines examined, the human colon CaCo-2 cell produces the most robust NO consumption activity. Cellular NO metabolism is Mostly O-2-dependent, produces near stoichiometric NO3-, and is inhibited by the heme, poisons CN-, CO (K-I approximate to 3 muM), phenylhydrazine, and NO and the flavoenzyme inhibitor diphenylene iodonium. NO consumption is saturable by O-2 and NO and shows apparent K-M values for O-2 and NO of 17 and 0.2 muM, respectively. Mitochondrial respiration, O-2(.-), and H2O2 are neither sufficient nor necessary for O-2-dependent NO metabolism by cells. The existence of an efficient mammalian heme and flavin-dependent NO dioxygenase is suggested. NO dioxygenation protects the NO-sensitive aconitases, cytochrome c oxidase, and cellular respiration from inhibition, and may serve a dual function in cells by limiting NO toxicity and by spatially coupling NO and O-2 gradients. (C) 2001 Elsevier Science Inc.