Proc. Nati. Acad. Sci. USA Vol. 88, pp. 10860-10864, December 1991 Biochemistry
Reversible conversion of nitroxyl anion to nitric oxide by superoxide dismutase (endothelium-derived relaxing factor/catalase/3-morpholinosydnonimine hydrochloride)
MICHAEL E. MURPHY* AND HELMUT SIESt Institut fur Physiologische Chemie I, Universitift Dusseldorf, Moorenstrasse 5, D-4000 Dusseldorf 1, Federal Republic of Germany
Communicated by Bruce N. Ames, August 16, 1991
Superoxide dismutase (SOD) rapidly scavABSTRACT enges superoxide (O°) and also prolongs the vasorelaxant effects of nitric oxide (NO), thought to be the endotheliumderived relaxing factor. This prolongation has been ascribed to prevention of the reaction between °2- with NO. We report that SOD supports a reversible reduction of NO to NO-. When cyanamide and catalase were used to generate NO- in the presence of SOD, NO was measured by the conversion of HbO2 to MetHb. When SOD[Cu(I)] was exposed to NO anaerobicafly, NO- was trapped by MetHb forming nitrosylmyoglobin. When NO was generated by 3-morpholinosydnonimine hydrochloride in the presence of SOD, NO- or a similar reductant was formed, which reduced catalase compound II and promoted the formation of the catalase[Fe(H)J-NO complex. It is, therefore, conceivable that SOD may protect NO and endothelium-derived relaxing factor by a mechanism in addition to O° scavenging and that NO- may be a physiologically important form of endothelium-derived relaxing factor.
Nitric oxide is thought to be the endothelium-derived relaxing factor (EDRF), a vasodilator produced from arginine. The similar chemical properties of NO and EDRF, including their apparent reactivity with °2, support this proposal (1-4). The reaction of NO with O2 (reactions la and b) has been directly observed by using pulse-radiolysis (5), but evidence of the reaction of EDRF with °2 is indirect. Namely, superoxide dismutase (SOD) prolongs the effects of both EDRF and exogenous NO, whether or not there is a simultaneous addition of compounds thought to generate O°. This consistent effect of SOD has been attributed (1-4) to its known ability to catalyze 0° dismutation (6). NO + °2
4
ONOO-
ONOO-
[la]
NO0
[lb]
One report (5) suggests the rate constant for reaction la may be much slower than diffusion limited (-56 x 106 M-1 s-1 at 370C). Also, the intracellular concentration of °2 is estimated to be quite low [e.g., in liver
Reversible conversion of nitroxyl anion to nitric oxide by superoxide dismutase (endothelium-derived relaxing factor/catalase/3-morpholinosydnonimine hydrochloride)
MICHAEL E. MURPHY* AND HELMUT SIESt Institut fur Physiologische Chemie I, Universitift Dusseldorf, Moorenstrasse 5, D-4000 Dusseldorf 1, Federal Republic of Germany
Communicated by Bruce N. Ames, August 16, 1991
Superoxide dismutase (SOD) rapidly scavABSTRACT enges superoxide (O°) and also prolongs the vasorelaxant effects of nitric oxide (NO), thought to be the endotheliumderived relaxing factor. This prolongation has been ascribed to prevention of the reaction between °2- with NO. We report that SOD supports a reversible reduction of NO to NO-. When cyanamide and catalase were used to generate NO- in the presence of SOD, NO was measured by the conversion of HbO2 to MetHb. When SOD[Cu(I)] was exposed to NO anaerobicafly, NO- was trapped by MetHb forming nitrosylmyoglobin. When NO was generated by 3-morpholinosydnonimine hydrochloride in the presence of SOD, NO- or a similar reductant was formed, which reduced catalase compound II and promoted the formation of the catalase[Fe(H)J-NO complex. It is, therefore, conceivable that SOD may protect NO and endothelium-derived relaxing factor by a mechanism in addition to O° scavenging and that NO- may be a physiologically important form of endothelium-derived relaxing factor.
Nitric oxide is thought to be the endothelium-derived relaxing factor (EDRF), a vasodilator produced from arginine. The similar chemical properties of NO and EDRF, including their apparent reactivity with °2, support this proposal (1-4). The reaction of NO with O2 (reactions la and b) has been directly observed by using pulse-radiolysis (5), but evidence of the reaction of EDRF with °2 is indirect. Namely, superoxide dismutase (SOD) prolongs the effects of both EDRF and exogenous NO, whether or not there is a simultaneous addition of compounds thought to generate O°. This consistent effect of SOD has been attributed (1-4) to its known ability to catalyze 0° dismutation (6). NO + °2
4
ONOO-
ONOO-
[la]
NO0
[lb]
One report (5) suggests the rate constant for reaction la may be much slower than diffusion limited (-56 x 106 M-1 s-1 at 370C). Also, the intracellular concentration of °2 is estimated to be quite low [e.g., in liver
