[Stability of a dinitrosyl iron complex with cysteine as a candidate for the role of blood vessel endothelial relaxation factor].

EPR evidence is presented that stability of the dinitrosyl iron complex (DNIC) with cysteine [(cys)2Fe(NO+)2] which determines its capability to produce NO depends on the redox state of the complex. The complex is maximally stable in the oxidized state characterized by the d6 electron configuration of iron. The reducing effect of dithionite, cysteine or GSH on this particular form of DNIC results in its transformation into unstable paramagnetic, NO-releasing forms. The effect of thiols is concentration-dependent. At low cysteine concentrations (1-5 mM and 20 microM of DNIC), thiols destabilize DNIC, acting as reducing agents. However, higher thiol concentrations (10 mM) added to DNIC stabilize the reduced forms of the complex, acting as ligands. The data obtained suggest that those authors who studied the vasodilatory activity of DNIC on isolated blood vessels, dealt with a stable oxidized form of DNIC. This form may arise due to oxidation of the unstable paramagnetic form of DNIC (DNIC 1:20) resulting from dilution of its aqueous solutions.