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Analysis of the blocking activity of charybdotoxin homologs and iodinated derivatives against Ca2+-Activated K+ channels

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Summary

Two charybdotoxin peptides were purified from venom of the Israeli scorpion,Leiurus quinquestriatus hebraeus. Microsequencing of the most abundant toxin, ChTX-Lq1, revealed identity with the 37-residue peptide previously sequenced by Gimenez-Gallego et al. [Gimenez-Gallego, G., et al.,Proc. Natl. Acad. Sci. USA 85:3329–3333 (1988)]. Sequence data on the minor peptide, ChTX-Lq2, showed substantial homology to ChTX-Lq1 with differences observed at eight positions. These two charybdotoxin sequences, along with that of noxiustoxin, define a distinct family of scorpion peptide toxins with activity against K+ channels. Both charybdotoxin homologs inhibited Ca2+-dependent K+ efflux from human erythrocytes with similar potency,K 0.5∼-40nm. In planar bilayer assays of single K(Ca) channels from rat muscle, ChTX-Lq1 and ChTX-Lq2 blocked with intrinsicK d's of 1.3 and 43nm, respectively, in the presence of 50mm external KCl. A new application of dwell-time histogram analysis of single-channel blocking events was used to characterize the kinetic homogeneity of toxin samples and the blocking kinetics of ChTX derivatives. The lower blocking affinity of ChTX-Lq2 was the combined result of a faster dissociation rate and a slower association rate as compared to ChTX-Lq1. The blocking activity of two mono-iodinated derivatives of ChTX-Lq1 was also analyzed. Blocked dwell-time histograms of the iodinated peptides were characterized by predominately brief (0.2–2 sec) blocking events in comparison to the native toxin (20 sec). Histogram analysis revealed that mono-iodination of ChTX-Lq1 impairs blocking activity by adverse effects on both dissociation and association rate constants. Frequency density histograms of single channel blocking events provide a sensitive assay of toxin purity suitable for quantitating structure-activity relationships of charybdotoxin derivatives.

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Authors and Affiliations

  1. Department of Pharmacology and Department of Cellular and Molecular Physiology, Yale University School of Medicine, 06510, New Haven, Connecticut

    Kathryn Lucchesi, Arippa Ravindran, Howard Young & Edward Moczydlowski

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  1. Kathryn Lucchesi
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  2. Arippa Ravindran
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  3. Howard Young
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  4. Edward Moczydlowski
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Lucchesi, K., Ravindran, A., Young, H. et al. Analysis of the blocking activity of charybdotoxin homologs and iodinated derivatives against Ca2+-Activated K+ channels. J. Membrain Biol. 109, 269–281 (1989). https://doi.org/10.1007/BF01870284

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  • DOI: https://doi.org/10.1007/BF01870284

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