Interactions of cytochromec with phospholipid membranes : Reactivity of cytochromec bound to phospholipid liquid crystals.

Cytochromec can be bound to mixed cardiolipin-lecithin liquid crystals so that it cannot be removed by repeated washings with solutions of high ionic strength. The oxidized and reduced spectra of this cytochromec show no detectable differences from those for soluble cytochromec. Unlike soluble cytochromec, however, some 90% of the bound cytochromec is not reduced by ascorbate, and it is only slowly reduced by dithionite. The addition of redox dyes causes complete and immediate reduction in the presence of ascorbate or dithionite. It is suggested that this is because the dyes possess some degree of lipid solubility and are able to penetrate the phospholipid membrane barriers separating cytochromec from the bulk solution. The addition of detergents, such as Triton X-100, also promotes reduction of the bound cytochromec by ascorbate. A small change in the standard potential from, 273 mV for soluble cytochromec to 225 mV for the bound cytochromec was found. The bound cytochromec reacts readily with potassium cyanide to form the normal cyanide-ferricytochromec complex, differing in the rate of formation from soluble cytochromec only at alkaline pH values. The relationship of these findings to work on the membrane-bound cytochromec in mitochondria and submitochondrial particles is discussed.