Competitive binding interaction between Zn2+ and saxitoxin in cardiac Na+ channels. Evidence for a sulfhydryl group in the Zn2+/saxitoxin binding site

Biophys J. 1991 Mar;59(3):523-37. doi: 10.1016/S0006-3495(91)82269-X.


Mammalian heart Na+ channels exhibit approximately 100-fold higher affinity for block by external Zn2+ than other Na+ channel subtypes. With batrachotoxin-modified Na+ channels from dog or calf heart, micromolar concentrations of external Zn2+ result in a flickering block to a substate level with a conductance of approximately 12% of the open channel at -50 mV. We examined the hypothesis that, in this blocking mode, Zn2+ binds to a subsite of the saxitoxin (STX) binding site of heart Na+ channels by single-channel analysis of the interaction between Zn2+ and STX and also by chemical modification experiments on single heart Na+ channels incorporated into planar lipid bilayers in the presence of batrachotoxin. We found that external Zn2+ relieved block by STX in a strictly competitive fashion. Kinetic analysis of this phenomenon was consistent with a scheme involving direct binding competition between Zn2+ and STX at a single site with intrinsic equilibrium dissociation constants of 30 nM for STX and 30 microM for Zn2+. Because high-affinity Zn2(+)-binding sites often include sulfhydryl groups as coordinating ligands of this metal ion, we tested the effect of a sulfhydryl-specific alkylating reagent, iodoacetamide (IAA), on Zn2+ and STX block. For six calf heart Na+ channels, we observed that exposure to 5 mM IAA completely abolished Zn2+ block and concomitantly modified STX binding with at least 20-fold reduction in affinity. These results lead us to propose a model in which Zn2+ binds to a subsite within or near the STX binding site of heart Na+ channels. This site is also presumed to contain one or more cysteine sulfhydryl groups.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Binding, Competitive
  • Biophysical Phenomena
  • Biophysics
  • Cattle
  • Dogs
  • In Vitro Techniques
  • Iodoacetamide / pharmacology
  • Kinetics
  • Molecular Sequence Data
  • Myocardium / metabolism*
  • Saxitoxin / metabolism
  • Sodium Channels / drug effects
  • Sodium Channels / genetics
  • Sodium Channels / metabolism*
  • Zinc / metabolism


  • Sodium Channels
  • Saxitoxin
  • Zinc
  • Iodoacetamide