Evidence for nonlinear capacitance in biomembrane channel system

The electrophysiological properties of voltage-dependent anion channels fro m mitochondrial membrane have been studied in a bilayer membrane system. It was observed that the probability of opening of the membrane channel depen ds on externally applied voltage and the plot is a bell-shaped curve symmet ric around probability axis. A scheme of conformational energy levels under varying externally applied voltage was formulated. Assuming that the proba bility follows Boltzmann distribution, we arrive at an expression of change in energy containing a separate term identical to the energy of a capacito r. This fact indicates the possibility of existence of an added capacitance due to the channel protein. Further it was shown that the aforesaid channe l capacitor could be a function of voltage leading to nonlinearity. We have offered a general method of calculating nonlinear capacitance from the exp erimental data on opening probability of a membrane channel. In case of vol tage-dependent anion channel the voltage dependence of the capacitor has a power 0.786. The results have been interpreted in view of the structural or ganization of the channel protein in the membrane. Our hypothesis is that t he phenomenon of capacitor behaviour is a general one for membrane channels . (C) 1999 Academic Press.