ON THE STABILITY OF THE DINITROSYL-IRON-CYSTEINE COMPLEX, A CANDIDATEFOR THE ENDOTHELIUM-DERIVED RELAXATION FACTOR

The stability of the dinitrosyl-iron-cysteine complex (DNIC) [(cys)(2) Fe(NO4)(2)], which determines its potency as an NO-producer in aqueous medium, was shown by EPR to be redox-dependent. The oxidized diamagne tic DNIC with electron configuration d(6) (Fe2+) has the highest stabi lity. It is reduced to unstable paramagnetic NO-releasing forms by dit hionite, cysteine, or GSH; the destabilizing effect of thiols is conce ntration-dependent. Low thiol concentrations (1-5 mM and 20 mu M DNIC) act as destabilizing reducing agents, whereas higher thiol concentrat ions act as ligands stabilizing the reduced forms of the complex. This suggests that Feelisch et al. (Nature, 368, 62-65 (1994)) in their ex periments on the vasodilatory activities of DNIC in isolated blood ves sels used a stable oxidized form of DNIC. This form might result from oxidation of the unstable paramagnetic DNIC (DNIC 1:20) after dilution of its aqueous solution.