Photochemical Identification of the Binding Region for (S)-Semotiadil on Sodium Channels: Comparison with That for (R)-Semotiadil on Skeletal Muscle Calcium Channel

Abstract

To clarify the recognition site of (S)-semotiadil in the Na + channels, we employed photoaffinity labeling with (S)-[ 3 H]-D51-4700, an azidophenyl derivative of (S)-semotiadil, to Na + channel preparations from rat brains and porcine hearts, and the results were compared with those by (R)-enantiomer to skeletal muscle Ca 2+ channel. Protease-digestion of the photolabeled Na + channels followed by quantitative immunoprecipitation assay using a site-directed antibody, revealed that 10-and 13-kDa fargments of brain and cardiac Na + channels, respectively, were located within the regions including IVS6. These regions are corresponding to the 8.3 kDa fragment photolabled by (R)-[ 3 H]-D51-4700 in the skeletal muscle Ca 2+ channel. Interestingly, however, the photoincorporation of (S)-[ 3 H]-D51-4700 into the Na + channel was 2-3 times higher than that of (R)-isomer; and visa versa in the Ca 2+ channels.