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. Hg
2+, 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 Hg
2+ block of hH1 (Time
50% = 179 s). L-cysteine did not prevent Hg
2+ 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 Hg
2+ block of hH1. L-cysteine did not prevent the Cd
2+ block of hH1. These results indicate that L-cysteine selectively acts on heart-specific Cys
373 in the P-loop region of hH1 to prevent Cys
373 from the oxidation-induced sulfur-Hg-sulfur bridge formation. (
Circ J 2002;
66: 846 - 850)
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