Mechanism of depression in cardiac sarcolemmal Na+-K+-ATPase by hypochlorous acid.

Oxidative stress during pathological conditions such as ischemia-reperfusion is known to promote the formation of hypochlorous acid (HOCl) in the heart and to result in depression of cardiac sarcolemmal (SL) Na+-K+-ATPase activity. In this study, we examined the direct effects of HOCl on SL Na+-K+-ATPase from porcine heart. HOCl decreased SL Na+-K+-ATPase activity in a concentration- and time-dependent manner. Characterization of Na+-K+-ATPase activity in the presence of different concentrations of MgATP revealed a decrease in the maximal velocity ( V max) value, without a change in affinity for MgATP on treatment of SL membranes with 0.1 mM HOCl. The V max value of Na+-K+-ATPase, when determined in the presence of different concentrations of Na+, was also decreased, but affinity for Na+ was increased when treated with HOCl. Formation of acylphosphate by SL Na+-K+-ATPase was not affected by HOCl. Scatchard plot analysis of [3H]ouabain binding data indicated no significant change in the affinity or maximum binding capacity value for ouabain binding following treatment of SL membranes with HOCl. Western blot analysis of Na+-K+-ATPase subunits in HOCl-treated SL membranes showed a decrease (34 ± 9% of control) in the β1-subunit without any change in the α1- or α2-subunits. These data suggest that the HOCl-induced decrease in SL Na+-K+-ATPase activity may be due to a depression in the β1-subunit of the enzyme.