Oxidative conversion of thiocyanate to cyanide by oxyhemoglobin during acid denaturation

Human oxyhemoglobin (Hb-O2) converts thiocyanate to cyanide under acidic conditions, in a hemoglobin and thiocyanate concentration-dependent manner. Other naturally occurring hemoglobin derivatives and hematin did not result in cyanide-producing activity. Pre-denaturation completely destroyed cyanide production. In the presence of 10% phosphoric acid, 12 microM cyanide was produced by 1 mM Hb-O2 (per heme) and 1 mM thiocyanate. This conversion was observed over a wide range of acidic pH levels at 50 degrees C, with an optimal pH of 4. The production of cyanide under strong acidic conditions was completed within 1 s, while under weak acidic conditions (pH approximately 5) the production gradually increased for half an hour. Under both conditions, the time course for cyanide production coincided with the disappearance of this converting activity. Thiocyanate conversion by acidified Hb-O2 was completely suppressed by the addition of ascorbic acid, significantly suppressed by superoxide anion radical (O2-) scavengers or thiourea, but not effected by scavengers of hydroxyl radical, singlet oxygen, and protein radical. In addition to acidification, other processes for denaturation of Hb-O2 resulted in detectable production of cyanide, e.g., heating, extraction by organic solvents, and incubation with denaturating or oxidative reagents. These data suggest that O2- generated during Hb-O2 denaturation and also a hydroperoxyl radical converted under acidic conditions are the oxidizing species that convert thiocyanate to cyanide.