Pulse Radiolysis Studies of the Reactions of CO3•- and NO2• with Nitrosyl(II)myoglobin and Nitrosyl(II)hemoglobin

Abstract

The reactions of carbonate radical anion [CO3•-, systematic name: trioxidocarbonate(•1−)] with nitrosyl(II)hemoglobin (HbFeIINO) and nitrosyl(II)myoglobin (MbFeIINO) were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature. The reactions proceed in two steps: outer-sphere oxidation of the nitrosyliron(II) proteins to their corresponding nitrosyliron(III) forms and subsequent dissociation of NO•. The second-order rate constants measured for the first reaction steps were (4.3 ± 0.2) × 108 and (1.5 ± 0.3) × 108 M-1 s-1, for MbFeIINO and HbFeIINO, respectively. The reactions between nitrogen dioxide and MbFeIINO or HbFeIINO were studied by pulse radiolysis in N2O-saturated 0.1 M phosphate buffer pH 7.4 containing 5 mM nitrite. Also for the reactions of this oxidant with the nitrosyliron(II) forms of Mb and Hb a two-step reaction was observed: oxidation of the iron was followed by dissociation of NO•. The second-order rate constants measured for the first reaction steps were (2.9 ± 0.3) × 107 and (1.8 ± 0.3) × 107 M-1 s-1, for MbFeIINO and HbFeIINO, respectively. Both radicals appear to be able to oxidize the iron(II) centers of the proteins directly. Only for the reactions with HbFeIINO it cannot be excluded that, in a parallel reaction, CO3•- and NO2• first react with amino acid(s) of the globin, which then oxidize the nitrosyliron(II) center.