Calcium-binding properties of the mitochondrial channel-forming hydrophobic component

A hydrophobic, low-molecular weight component extracted from mitochondria forms a Ca²⁺-activated ion channel in black-lipid membranes (Mironova et al., 1997). At pH 8.3-8.5, the component has a high-affinity binding site for Ca²⁺ with a K(d) of 8 X 10^-6 M, while at pH 7.5 this K(d) was decreased to 9 X 10^-5 M. B(max) for the Ca²⁺-binding site did not change significantly with pH. In the range studied, 0.2 ± 0.06 mmol Ca²⁺/g component were bound or one calcium ion to eight molecules of the component. The Ca²⁺ binding was strongly decreased by 50-100 mM Na⁺, but not by K⁺. Treatment of mitochondria with CaCl₂ prior to ethanolic extraction resulted in a high level of Ca²⁺-binding capacity of the partially purified component. Cyclosporin A, a specific inhibitor of the mitochondrial permeability transition, when added to the mitochondrial suspension, decreased the Ca²⁺-binding activity of the purified extract severalfold. The calcium-binding capability of the partially purified component correlates with its calcium-channel activity. This indicates that the channel-forming component might be involved in the permeability transition that stimulates its formation.