[Dinitrosyl complexes of iron with thiol-containing ligands and their reverse conversion into nitrosothiols]

Bovine serum albumin (BSA) is capable of forming dinitrosyl complexes with iron (DNIC) containing one thiol and one nonthiol ligand and yielding an EPR signal at the following values of the g-factor: g1 = 2.046; g2 = 2.03; g3 = 2.012. During interaction with L-cysteine or N-acetyl-L-cysteine the complex symmetry increased due to the substitution within DNIC of the nonthiol ligand of BSA for cysteine; such DNIC was characterized by an EPR signal with an axial-symmetrical tensor of the g-factor. At high cysteine concentrations all of the Fe(NO)-groups appeared to be transferred from BSA to cysteine to form DNIC with cysteine yielding an EPR signal with a g-factor of g perpendicular = 2.037; g parallel = 2.012. The lifetime of DNIC-BSA was about 24 hrs, whereas that of DNIC-cysteine was less than 1 min due to cysteine oxidation in the air. In 0.5 M HCl DNIC-BSA and DNIC-cysteine were reversibly converted into appropriate nitrosothiols characterized by intensive adsorption at 340-360 nm. Upon subsequent neutralization of the solution and addition of the substituent (cysteine or dithionite) these nitrosothiols were converted into DNIC. In the absence of iron cysteine and dithionite caused reductive destruction of protein and low molecular weight nitrosothiols to liberate nitrogen oxide. This property of nitrosothiols makes them distinct from those DNIC, in which cysteine acts exclusively as a scavenger of Fe(NO)-groups by shifting the equilibrium between the protein and low molecular weight DNIC towards the latter.