ROLES OF IRON IONS AND CYSTEINE IN FORMATION AND DECOMPOSITION OF S-NITROSOCYSTEINE AND S-NITROSOGLUTATHIONE

The decomposition of S-nitrosocysteine (cys-NO) in aqueous solutions a t neutral pH is catalyzed by traces of iron ions. Desferal, which sele ctively binds iron ions, dramatically inhibits the decomposition of cy s-NO. Cysteine and reduced glutathione (GSH) had a similar effect; the y formed dinitrosyl iron complexes (DNIC) with contaminating iron ions , NO molecules, and NO+ ions liberated from cys-NO. Addition of Fe2+ t o cys-NO in the presence of cysteine decomposed cys-NO and formed the DNIC. The effect of cysteine on cys-NO was opposite to its effect on S -nitrosoglutathione (GS-NO). Cysteine caused decomposition of GS-NO. E xogenous iron accelerated this decomposition by the formation of the D NIC. The decomposition of GS-NO may result from S-transnitrosylation f rom GSH to cysteine and formation of the unstable cys-NO. The DNICs ar e converted to GS-NO during decomposition of DNIC catalyzed by citrate , which removes iron from the DNIC. Thus, in solutions of cysteine or GSH, the S-nitrosothiols and DNICs can be interconverted. In the prese nce of iron, the S-nitrosothiols are converted to the DNICs. Removal o f iron from the DNICs generates the S-nitrosothiols. These processes m ay be important to the physiological activities of DNICs and S-nitroso thiols as sources of NO.