Interaction of Biguanides with Mitochondrial and Synthetic Membranes

Abstract

Biguanides are inhibitors of oxidative phosphorylation which can be accumulated by isolated mitochondria. The interaction of rat liver mitochondria, submitochondrial particles, and of synthetic phospholipid vesicles with biguanides of different lipophilic properties has been investigated. Binding has been studied by use of 14C‐labelled biguanides, as well as by fluorescence changes of the polarity sensitive membrane probe 1‐anilino‐8‐naphthalene sulfonate. Liberation of protons during biguanide binding was mentioned spectrophotometrically with bromocresol purple. The following properties of biguanide binding have been identified. The biguanides bind to the membrane in a rapid reaction, at least one order of magnitude faster than onset of inhibition. During this initial phase of binding, protons are released from the membrane. The affinity constants for binding are mainly determined by the degree of hydrophobic interaction and are directly correlated with the partition coefficients. Evidence is derived that all biguanide present in mitochondria is in a bound state. Binding is influenced in an opposite way by energization of mitochondria versus submitochondrial particles. From the effects of energy, changes of pH, presence of K+ or K+ with valinomycin, it is suggested that electrostatic contribution to binding is modified by the membrane potential rather than by a pH gradient. The characteristics of binding to natural membranes can be widely simulated by synthetic phospholipid vesicles, suggesting that membrane phospholipids represent the natural binding site. Copyright © 1972, Wiley Blackwell. All rights reserved