Hemoglobin hydroxylamine oxidation

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. 

The first report on the production of an hydroxylamine metabolite from a nitroaromatic or other xenobiotic compound was in 1944, when 2,6-dinitro-4-hydroxylaminotoluene was obtained in crystalline form from the urine of rabbits treated with 2,4,6-trinitrotoluene (TNT) (7). The well-known ability of TNT to induce methemoglobinemia was rationalized by the ability of this metabolite to react chemically with oxyhemoglobin and cause oxidation of the hemoglobin iron. TNT was a fortuitous choice because its hydroxylamine metabolite is among the most stable known. Most nitroso and hydroxylamine metabolites defy direct isolation from in vivo studies because of their chemical reactivity. In addition, they are rarely found as metabolites in bile or urine, except for some conjugated forms of the arylhydroxylamine compounds. 

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