A hydrophobic, low-molecular weight component extracted from mitochondria f
orms 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 boun
d 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 mitoch
ondria 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 add
ed to the mitochondrial suspension, decreased the Ca2+-binding activity of
the purified extract severalfold. The calcium-binding capability of the par
tially purified component correlates with its calcium-channel activity. Thi
s indicates that the channel-forming component might be involved in the per
meability transition that stimulates its formation.