CONDITIONS LIMITING THE USE OF IONOPHORE A23187 AS A PROBE OF DIVALENT-CATION INVOLVEMENT IN BIOLOGICAL REACTIONS - EVIDENCE FROM THE SLOW FLUORESCENCE QUENCHING OF TYPE-A SPINACH-CHLOROPLASTS

The conditions under which ionophore A23187 can be used as a probe of Mg2+ involvement in the reactions of intact (Type A) spinach chloroplasts have been investigated by monitoring ionophore-induced reversal of slow fluorescence quenching. The following observations were made: (1) A23187-dependent reversal of quenching is a strong function of pH. This is consistent with competition between protons and divalent cations for the carboxylic acid moiety of the ionophore. (2) In the presence of exogenous Mg2+, quenching reversal by A23187 is significantly slowed. It is suggested that formation of the dimeric A23187 · Mg2+ complex delays action of the ionophore at the thylakoid membrane by slowing equilibration of the ionophore among chloroplast membrane phases. (3) In the absence of Mg2+, significant interaction of A23187 with certain monovalent cations - Li+ and Na+, but not K+ - is observed. Evaluations of the interaction of ionophore A23187 with specific biological systems and inferences of divalent cation involvement, or lack thereof, must take these limitations into account. © 1979.