Effects of NOx compounds on heme-containing enzymes

Nitric oxide (NO) has been shown to be involved in many important biological processes. The biochemical pathways of these processes share two common features: the enzymatic synthesis of NO from L-arginine and the formation of an iron-nitrosyl complex in a target (heme) protein to evoke the function of purpose. The odd electron of NO causes the heme-nitrosyl compounds to become paramagnetic and thus detectable by Electron Paramagnetic Resonance (EPR) Spectroscopy. Heme-nitrosyl EPR signals have been observed in tumors from various sources. Such signals were recently observed to be products of the cell-mediated immune response to tumor cells in vivo. In an attempt to determine the origin of these signals, we tested the effects of various NO donors on several heme-containing proteins in vitro. The NO donors tested include NO (g), NO- in the form of Angelas salt, NO+ in the form of NOBF4, and S-nitrosoglutathione (GSNO) which may break down into all three previous NO forms. Peroxynitrite anion (ONOO) was also tested since it has close relationship to NO chemistry and it is increasingly recognized as a biologically generated reactive species that is derived from NO-. Furthermore, effects of nitrite (N02)and nitrate anions (NO3) were also examined. With the heme-containing enzymes tested, NO- inhibited most of them except for myeloperoxidase which is considered to be different in its heme structure from the other enzymes. ONOO- was observed to strongly inhibit all the enzymes which implies that it does play an important role in heme-NO reactions. On the other hand, NO-, NO+, and GSNO had little or no effects on both enzymatic activities and the optical spectra of the enzymes. We expect to determine the NO binding patterns once the EPR spectra are available.