Hypochlorous acid-induced mobilization of zinc from metalloproteins

Hypochlorous acid (HOCl), a neutrophil oxidant, can contribute to tissue injury at sites of inflammation by its reactivity with protein sulfhydryls. The present study shows that physiological concentrations (50-200 microM) of HOCl can displace Zn2+ from metalloproteins, such as metallothionein and alcohol dehydrogenase, in which the metal is bound to sulfhydryls by means of thiolate (S-Zn) bonds. No mobilization of Zn2+ was observed from superoxide dismutase in which the metal is not bound to cysteine, suggesting that HOCl reacts selectively with thiolate bonds. Zn2+ mobilization, measured spectrophotometrically with the metallochromic indicator 4-(2-pyridylazo)resorcinol, was also observed from complexes of this metal with other thiol-containing compounds such as 2,3-dimercaptopropanol and metallothionein fragment 56-61. HOCl cleavage of the thiolate bonds was confirmed by the decrease in absorbance at 250 nm. This study shows for the first time that HOCl can mobilize protein-bound Zn2+ and suggests that neutrophil oxidant injury may be partially mediated by the mobilization of cellular Zn2+.