Evidence against formation of A23187 dimers and oligomers in solution: Photo-induced degradation of ionophore A23187

Timothy P. Thomas, Exing Wang, Douglas R. Pfeiffer, Richard W. Taylor

Research output: Contribution to journalArticlepeer-review

7 Scopus citations

Abstract

Ionophore A23187 has been proposed to form Ca2+ conducting channels that arise from dimers and oligomers of the compound (e.g., Balasubramanian, S. V., and Easwaran, K. R. K. (1989) Biochem. Biophys. Res. Commun. 158, 891- 897). To investigate this possibility, the solution behavior of A23187 in chloroform, n-hexane, ethanol, 80% methanol-water, and palmitoyloleoylphosphatidyl choline (POPC) vesicles was investigated using UV-VIS, circular dichroism (CD), and 1H NMR techniques. The concentration dependence of the UV-VIS and CD spectra obtained in freshly prepared chloroform solutions indicates that neutral A23187 (HA) exists as a monomer for ionophore concentrations in the range of 50-1000 μM. The cause of time- and concentration-dependent spectral alterations which gave rise to the dimer/channel hypothesis was also investigated. For solutions of 50-1000 μM A23187 in chloroform, n-hexane, and ethanol stored in the dark, no spectral changes were observed for periods of 2 months. However, solutions in these solvents did show time-dependent spectral changes when exposed to light. In 80% methanol-water or phospholipid vesicles, similar spectral changes were observed, even when the solutions were protected from light. Application of TLC and MS methods indicate that the time-dependent spectral changes reflect degradation of A23187, not dimer or oligomer formation. The degradative processes proceed with half-lives ranging from ~75 to >400 h, and are influenced by several factors, including solvent, exposure to light, ionophore concentration, pH, and the presence of metal ions, EDTA, dissolved oxygen, and a radical inhibitor. The kinetics of Ca2+ transport into Quin- 2-loaded POPC vesicles by authentic A23187 give no evidence of a channel mechanism, even following a previous and lengthy coincubation of the ionophore with the vesicles.

Original languageEnglish (US)
Pages (from-to)351-361
Number of pages11
JournalArchives of Biochemistry and Biophysics
Volume342
Issue number2
DOIs
StatePublished - Jun 15 1997
Externally publishedYes

Keywords

  • Ca transport
  • Ion- conducting channels
  • Ionophore A23187
  • Phospholipid vesicles
  • Photo-induced degradation

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Molecular Biology

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