Role of redox‐active iron ions in the decomposition of S‐nitrosocysteine in subcellular fractions of porcine aorta

We recently reported that degradation of S‐nitrosocysteine in homogenates of porcine aorta increased severalfold in the presence of Mg2+ ions [Kostka, P., Xu, B. & Skiles, E.H. (1999) J. Cardiovasc. Pharmacol.33, 665–670]. The objective of the present study was to examine this in greater detail. The rate of S‐nitrosocysteine degradation by aortic homogenates in the presence of Mg2+ ions exhibited differential sensitivity to chelators of iron ions. Terpyridine and diethylenetriamine penta‐acetic acid (5–500 µm) caused a concentration‐dependent inhibition of S‐nitrosocysteine decay, whereas deferoxamine (100 µm) was ineffective. o‐Phenanthroline (250 µm), a selective chelator of Fe2+ ions, potentiated the reaction at low initial concentrations of S‐nitrosocysteine (≤ 15 µm) and inhibited the reaction at higher concentrations. The inhibitory effects of o‐phenanthroline were related to suppression of S‐nitrosocysteine decay by cysteine‐mediated reduction of Fe3+. In the presence of o‐phenanthroline, S‐nitrosocysteine decomposition followed saturable kinetics with K0.5 = 3.8 ± 0.3 µm and h = 1.8 ± 0.1 (mean ± SE, n = 4). Comparison of the rates of S‐nitrosocysteine decay in different subcellular fractions showed selective association with the cytosolic fraction, as documented by copurification with lactate dehydrogenase activity. At non‐limiting concentrations of S‐nitrosocysteine, the rate of degradation in the cytosolic fraction was 4.1 ± 0.3 nmol·min−1·(mg protein)−1 (n = 4). It is concluded that the cytosolic fraction of porcine aorta contains a protein factor, presumably an enzyme, capable of catalyzing heterolytic decomposition of the S–NO bond of S‐nitrosocysteine in a process involving redox cycling of iron ions.