Interaction of biguanides with mitochondrial and synthetic membranes. Effects on ion conductance of mitochondrial membranes and electrical properties of phospholipid bilayers.

Abstract

Primary reactions of the energy‐conserving pathway in oxidative phosphorylation are inhibited by biguanides and alkylguanidines. The inhibitors bind to phospholipids of mitochondria and synthetic membranes with identical affinities and are assumed to modify the physical properties of membranes, as for example cation conductance.The study compares effects of biguanides on respiratory‐linked ion fluxes across mitochondrial membranes with those on the conductance and the electrical capacitance of phospholipid bilayers. The following observations have been made:1. The rate of energy‐linked mitochondrial swelling and the rate of proton uptake during phosphorylation of ADP is inhibited in presence of biguanides. The activity of different compounds is proportional to their binding affinity to phospholipids.2. Oscillatory volume changes of mitochondria are largely abolished by biguanides.3. Biguanides cause a small increase of electrical conductance of phospholipid bilayers. This is only in part due to charge transfer by the lypophilic biguanide cations. It is suggested that also structural changes are induced by biguanides altering the surface area per molecule of phospholipid. At high biguanide concentrations structural reorganization is also expressed by changes in electrical capacitance. Both effects are saturable and proportional to binding affinity.4. Biguanides affect the electrical conductance of bilayers in the opposite way but to the same extent with positively charged species (K+/ionophore) and with negatively charged species (uncouplers of oxidative phosphorylation). This is explained by generation of a positive surface potential at phospholipid containing membranes.5. The data fit excellently the application of diffuse double‐layer theory to lipid bilayers. The relative effectiveness of different biguanides on model membranes is identical to that on ion fluxes with mitochondrial membranes. From this it is concluded that a similar mechanism of action applies to synthetic and natural membranes as well, suggesting that screening of functional negative sites interferes with the translocation of cations linked to electron transport and the primary reactions in conservation of respiratory energy.