Cation transport properties of a synthetic Ca 2+-selective peptide ionophore in phospholipid and sarcoplasmic reticulum vesicles

Abstract

Transport by the synthetic cyclic peptide ionophore CYCLEX-2E (Deber, C.M., Young, M.E.M., and Tom-Kun, J. (1980) Biochemistry 19, 6194–6198), which in contrast to Ca 2+ ionophore A23187 contains no ionizable protons, has been studied with respect to Ca 2+ and Na + transport, and the involvement of exchanged, or counter-transported ions during the transport process. CYCLEX-2E was found to equilibrate Na + and Ca 2+ gradients across phospholipid vesicle membranes. Experiments using the indicator dye Arsenazo III established that calcium ions were indeed reaching the aqueous intravesicular compartments. Absence of metal cations in the external buffer slowed, but did not eliminate, the efflux of Ca 2+ from phosphatidylcholine vesicles. As an example of its activity in a biological membrane, CYCLEX-2E was shown to be capable of producing Ca 2+ efflux from sarcoplasmic reticulum vesicles which had been loaded with Ca 2+ in an ATP-dependent manner. The overall results suggest that in transport by synthetic peptide ionophores typified by CYCLEX-2E, electroneutrality is achieved either through (a) peptide-mediated compensating (but not coupled) fluxes of other cations, or where this is not an option, by (b) transmembrane diffusion of permeant ions such as H +, OH −, or Cl −.