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A hydrophobic, low-molecular weight component extracted from mitochondria forms aCa2+-activated ion channel in black-lipid membranes (Mironova et al., 1997). At pH 8.3–8.5, thecomponent has a high-affinity binding site for Ca2+ with a Kd of 8 × 10−6 M, while at pH7.5 this Kd was decreased to 9 × 10−5 M. Bmax for the Ca2+-binding site did not changesignificantly with pH. In the range studied, 0.2 ± 0.06 mmol Ca2+/g component were boundor one calcium ion to eight molecules of the component. The Ca2+ binding was stronglydecreased by 50–100 mM Na+, but not by K+. Treatment of mitochondria withCaCl2 priorto ethanolic extraction resulted in a high level of Ca2+-binding capacity of the partially purifiedcomponent. Cyclosporin A, a specific inhibitor of the mitochondrial permeability transition,when added to the mitochondrial suspension, decreased the Ca2+-binding activity of thepurified extract severalfold. The calcium-binding capability of the partially purified componentcorrelates with its calcium-channel activity. This indicates that the channel-forming componentmight be involved in the permeability transition that stimulates its formation. |
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