Hypochlorous acid inhibits Ca(2+)-ATPase from skeletal muscle sarcoplasmic reticulum

Hypochlorous acid (HOCl) is produced by polymorphonuclear leukocytes that migrate and adhere to endothelial cells as part of the inflammatory response to tissue injury. HOCl is an extremely toxic oxidant that can react with a variety of cellular components, and concentrations reaching 200 microM have been reported in some tissues. In this study, we show that HOCl interacts with the skeletal sarcoplasmic reticulum Ca(2+)-adenosinetriphosphatase (ATPase), inhibiting transport function, HOCl inhibits sarcoplasmic reticulum Ca(2+)-ATPase activity in a concentration-dependent manner with a concentration required to inhibit ATPase activity by 50% of 170 microM and with complete inhibition of activity at 3 mM. A concomitant reduction in free sulfhydryl groups after HOCl treatment was observed, paralleling the inhibition of ATPase activity. It was also observed that HOCl inhibited the binding of the fluorescent probe fluorescein isothiocyanate to the ATPase protein, indicating some structural damage may have occurred. These findings suggest that the reactive oxygen species HOCl inhibits ATPase activity via a modification of sulfhydryl groups on the protein, supporting the contention that reactive oxygen species disrupt the normal Ca(2+)-handling kinetics in muscle cells.