Thermodynamic parameters for the reduction reaction of membrane-bound cytochrome c in comparison with those of the membrane-free form: spectropotentiostatic determination with use of an optically transparent thin-layer electrode.

An optically transparent thin-layer electrode cell with a very small volume has been developed to investigate the formal reduction potential (E0) of horse heart cytochrome c with and without cardiolipin-containing lecithin liposomes. Temperature-dependent experiments of membrane-free cytochrome c (10-40 degrees C) yielded standard thermodynamic parameters in agreement with literature values. On the other hand, membrane-bound cytochrome c had a break point(s) at the range between 10 and 30 degrees C when the formal reduction potential was displayed with respect to temperature. Both enthalpy and entropy changes had large negative values in the high-temperature region (greater than 30 degrees C) relative to those of the membrane-free form, suggesting membrane participation during the reduction course of cytochrome c. In addition, we observed an extrathermodynamic relationship between enthalpy and entropy values among different cytochrome c's and horse heart cytochrome c with and without membrane.