Nitrite metabolic generation

Figure 9. Metabolic generation of nitrite from a nitro compound in vitro, from...

Organisms also evolved powerful detoxifying mechanisms that remove toxic materials or convert them to non-toxic forms or nutrients. Examples of alterations to non-toxic forms are the conversions of hydrogen sulfide to sulfate and nitrite to nitrate. The prime example of development of the ability to use a toxic substance is the evolution of aerobic metabolism, which converted a serious and widespread toxin, oxygen, into a major resource. This development, as we have seen, greatly increased the productivity of the biosphere and generated the oxygen-rich atmosphere of today s Earth. 

The present consensus, therefore, is that, in the absence of unequivocal evidence that glutathione-S-transferase plays a major role in either bioconversion of GTN to NO or in GTN-induced vasorelaxation, it acts simply to catalyse generation of nitrite in this context, a non-productive competing metabolic route. 

Fung and colleagues examined the metabolic conversion of organic nitrates in sub-cellular fractions of bovine coronary artery smooth muscle cells [66, 67]. They found NO-generating capacity to be present in membrane fractions and, with the use of marker enzymes, identified plasma membrane as the primary location. The enzyme involved in bioconversion was not glutathione-S-transferase [68] and differed from those that catalyse activation of organic nitrites [69]. Partial purification [70] established that the molecular sizes of the native enzyme and subunits were approximately 200 kDa and 58 kDa respectively, and that enzymic activity depends on the presence of a free thiol group. 

Strueturally related adduets include the 8-phenol derivatives, 8-(4"-hydroxyphenyl)-dG (8-p-PhOH-dG), and 8-(2"-hydroxyphenyl)-dG (8-o-PhOH-dG) that are generated by reaetion of phenol with excess nitrite.This reaetion generates diazoquinones that break down into hydroxyphenyl radicals that attach covalently to C-8 of dG. Diazoquinones are mutagenic in the Ames test without metabolie aetivation, and it is expected that hydroxyphenyl radicals and the isomeric adducts, 8-p-PhOH-dG and 8-o-PhOH-dG, play a key role in the mutagenie activity of diazoquinones. Careinogenic PAHs, such as benzo[a]pyrene (BP), also form C8-dG adducts (8-BP-dG, Fig. 7). 2,173 hile classical metabolic activation of... 

The first product of nitrosyl transfer to nitrite in Eq. (2), E N203, contains N-N bonded N2O3 which is itself a well-known and powerful nitrosyl donor. It is reasonable to suppose therefore that nitrosyl transfer reactions with N- and O-nucleophiles could involve both E NO (or E HONO) and E N205. In addition, the involvement of a second molecule of nitrite for denitrification would require that the substrate saturation curve should be sigmoidal to reflect a term second-order in nitrite concentration. No such effect has been reported to our knowledge. The use of bimolecular substrate kinetics in dilute solutions to generate an intermediate subject to solvolysis seems metabolically unwise hut not impossible. 

Local nitrite concentrations in tissues are linked to the amounts of NO produced. Indeed, except for nitrate generated from the reaction of NO with oxyHb, nitrite is the major end product of NO metabolism. Increased nitrite levels are thus found under pathophysiological conditions, for example inflammation, when N O production is elevated. We have found that the rate constants for the reactions of the ferryl forms of Mb and Hb with nitrite are significantly lower than those for the corresponding reaction with NO (16 + 1 m 1 s 1 at pH 7.5 for Mb and (7.5 + 0.1) x 102 m 1 s 1 at pH 7.0 for Hb, at 20 °C) [19, 20]. Thus, the reaction with nitrite probably plays a role only when NO has been consumed completely and large concentrations of nitrite are still present. 

It is generally agreed that nitrate is the major source of inorganic N available to the leaves of most land plants ammonia is not considered to be present in the xylem stream leaving the roots (Pate, 1971). Thus the ammonia assimilated is normally generated in situ from the reduction of nitrate via nitrite. Consequently experiments in which ammonia is fed to detached leaves or parts of leaves probably do not fully reflect the path of the in vivo metabolism of nitrate to amino acids. 

It is well established that most of the known anaerobic prokaryotes perform oxidative phosphorylation without O2. Depending on the species and the metabolic conditions, these bacteria may use a large variety of inorganic (e.g., nitrate, nitrite, sulfate, thiosulfate, elemental sulfm, polysulfide sulfur) or organic compounds (e.g., fumarate, dimethylsulfoxide, trimethylamine-A -oxide, vinyl- or arylchlorides) as terminal electron acceptor instead of oxygen. The redox reactions with these acceptors are catalyzed by membrane-integrated electron transport chains and are coupled to the generation of an electrochemical proton potential (Ap) across the membrane. Oxidative phosphorylation in the absence of O2 is also termed anaerobic respiration . Oxidative phosphorylation with elemental sulfur is called sulfm respiration . Oxidative phosphorylation with polysulfide sulfur is called polysulfide respiration . 

---