Nitrite metabolism

Walters, C. L., and Taylor, A. M. (1964). Nitrite metabolism by muscle in vitro. Biochim. Biophys. Acta 86, 448-458. 

The iron-sulfur cluster is often present as the prosthetic group in oxidoreductases where the couple of the substrate is below 0 mV. This reflects the reducing power of the iron-sulfur cluster. There are, however, a small number of iron-sulfur-containing enzymes that function in the positive redox range. These are often associated with bacterial nitrate and nitrite metabolism. The least complicated enzymes contain no additional cofactor, while others contain a number of additional cofactors. 

Wada, E., and Hattori, A. (1971). Nitrite metabolism in the euphoric layer of the central North Pacific Ocean. Limnol. Oceanogr. 16, 766-772. 

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. 

Little is known regarding the pharmacokinetic properties of volatile nitrites in humans, particularly isobutyl nitrite and its primary metabolite, isobutyl alcohol. In rodents, after an intravenous infusion of isobutyl nitrite, blood concentrations peaked rapidly and then declined, with a half-life of 1.4 minutes and blood clearance rate of 2.9 L/min/kg (Kielbasa and Fung 2000). Approximately 98% of isobutyl nitrite is metabolized rapidly to isobutyl alcohol, concentrations of which also decline rapidly, with a half-life of 5.3 minutes. Bioavailability of inhaled isobutyl nitrite at a concentration of 300-900 ppm is estimated to be 43%. 

Toluene, volatile nitrites, and anesthetics, like other substances of abuse such as cocaine, nicotine, and heroin, are characterized by rapid absorption, rapid entry into the brain, high bioavailability, a short half-life, and a rapid rate of metabolism and clearance (Gerasimov et al. 2002 Pontieri et al. 1996, 1998). Because these pharmacokinetic parameters are associated with the ability of addictive substances to induce positive reinforcing effects, it appears that the pharmacokinetic features of inhalants contribute to their high abuse liability among susceptible individuals. 

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

Nitrite (or compounds at the same or lower oxidation level) is produced microbiologically from nitrate, and may then react with the substrate to produce stable end products. The production of nitrite is the sole metabolic function of the bacteria and, in view of concern over the presence of nitrate in groundwater, the following possible environmental significance of these or analogous reactions should not be overlooked ... 

Servent D, C Ducrorq, Y Henry, A Guissani, M Lenfant (1991) Nitroglycerin metabolism by Phanerochaete chrysosporium evidence for nitric oxide and nitrite formation. Biochim Biophys Acta 1074 320-325. 

The biotransformation of gylcerol trinitrate by strains of Bacillus thuringiensis/cereus or Enterobacter agglomerans (Meng et al. 1995), by strains of Pseudomonas sp., and some Entero-bacteriaceae (Blehert et al. 1997) involves the expected successive loss of nitrite with the formation of glycerol. The biotransformation of pentaerythritol tetranitrate by Enterobacter cloacae proceeds comparably with metabolism of two hydroxymethyl groups produced by loss of nitrite to the aldehyde (Binks et al. 1996). 

The primary function of the mammalian red blood cell is to maintain aerobic metabolism while the iron atom of the heme molecule is in the ferrous (Fe+2) oxidation state however, copper is necessary for this process to occur (USEPA 1980). Excess copper within the cell oxidizes the ferrous iron to the ferric (Fe+3) state. This molecule, known as methemoglobin, is unable to bind oxygen or carbon dioxide and is not dissociable (Langlois and Calabrese 1992). Simultaneous exposure of sheep to mixtures of cupric acetate, sodium chlorite, and sodium nitrite produced a dose-dependent increase in methemoglobin formation (Calabrese et al. 1992 Langlois and Calabrese 1992). 

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. 

As mentioned earlier, the therapeutic use of organic nitrites [4] actually predates that of organic nitrates [1], Clinical utilisation of nitrites has, however, been very much less and this is reflected in the relatively sparse attention given to their mechanisms of action. Alkyl nitrites react readily with thiols to form S-nitrosothiols [122], which show biological effects similar to those of NO [11]. Nevertheless, glutathione-S-transferase has been implicated in the metabolism of organic nitrites, via intermediate... 

T. A., Eisenthal, R., Harrison, R., Reduction of organic nitrites to nitric oxide catalysed by xanthine oxidase possible role in metabolism of nitrovasodilators. Biochem. Biophys. Res. Commun. 270 (2000), p. 880-885... 

Given that hydroxylamine reacts rapidly with heme proteins and other oxidants to produce NO [53], the hydrolysis of hydroxyurea to hydroxylamine also provides an alternative mechanism of NO formation from hydroxyurea, potentially compatible with the observed clinical increases in NO metabolites during hydroxyurea therapy. Incubation of hydroxyurea with human blood in the presence of urease results in the formation of HbNO [122]. This reaction also produces metHb and the NO metabolites nitrite and nitrate and time course studies show that the HbNO forms quickly and reaches a peak after 15 min [122]. Consistent with earlier reports, the incubation ofhy-droxyurea (10 mM) and blood in the absence of urease or with heat-denatured urease fails to produce HbNO over 2 h and suggests that HbNO formation occurs through the reactions of hemoglobin and hydroxylamine, formed by the urease-mediated hydrolysis of hydroxyurea [122]. Significantly, these results confirm that the kinetics of HbNO formation from the direct reactions of hydroxyurea with any blood component occur too slowly to account for the observed in vivo increase in HbNO and focus future work on the hydrolytic metabolism of hydroxyurea. 

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