L-Cysteine Prevents Oxidation-Induced Block of the Cardiac Na⁺ Channel Via Interaction With Heart-Specific Cysteinyl Residues in the P-Loop Region

Abstract

The present study investigated the protective effects of L-cysteine on the oxidation-induced blockade of Na⁺ channel α-subunits, hH1 (cardiac) and hSkM1 (skeletal), expressed in COS7 cells. Na⁺ currents were recorded by the whole-cell patch clamp technique (n = 3-7). L-cysteine alone blocked hH1 and hSkM1 in a dose-dependent manner, with saturating L-cysteine block at 3,000 μmol/L. Hg²⁺, a potent sulfhydryl oxidizing agent, blocked hH1 with a time to 50% inhibition (Time_50%) of 20 s. Preperfusion of COS7 cells with 100 μmol/L L-cysteine significantly slowed the Hg²⁺ block of hH1 (Time_50% = 179 s). L-cysteine did not prevent Hg²⁺ block of hSkM1 (Time_50% = 37 s) or the C373Y hH1 mutant (Time_50% = 43 s). As for other sulfo-amino acids, homocysteine prevented the Hg ²⁺ block of hH1, with the Time_50% (70 s) being significantly smaller than that of L-cysteine, whereas methionine did not prevent the Hg²⁺ block of hH1. L-cysteine did not prevent the Cd²⁺ block of hH1. These results indicate that L-cysteine selectively acts on heart-specific Cys³⁷³ in the P-loop region of hH1 to prevent Cys³⁷³ from the oxidation-induced sulfur-Hg-sulfur bridge formation. (Circ J 2002; 66: 846 - 850)