MITOCHONDRIAL ADP ATP CARRIER CAN BE REVERSIBLY CONVERTED INTO A LARGE CHANNEL BY CA2+/

Single-channel current measurements of excised patches with reconstitu ted purified mitochondrial ADP/ATP carrier (AAC) indicates the presenc e of a large low cation selective (P-K+/P-Cl- = 4.3 +/- 0.6) channel. The channel conductance has multiple sublevels and varies from 300 to 600 pS. It has low probability of current fluctuations at V-hold up to 80-100 mV of both signs and is reversibly gated at V-hold > 150 mV. T he opening of the channel is Ca2+-dependent (1 mM Ca2+) and can be rev ersibly closed on removal of Ca2+. It is strongly pH dependent and clo ses completely at pH(ex) 5.2. The AAC-specific inhibitor bongkrekate i nhibits the channel partially and completely in combination with ADP, whereas carboxyatractylate did not affect the conductance. The effects of these AAC-specific ligands prove that the channel activity belongs to AAC. The AAC-linked conductance can clearly be differentiated from the porin channel, rarely detected in our preparations. The propertie s of the AAC-linked channel coincide with the mitochondrial permeabili ty transition pore (MTP), which is also affected by the AAC ligands [H unter, D. R., & Haworth, R. A. (1979) Arch. Biochem. Biophys. 195, 453 -459] and resembles the mitochondrial ''multiconductance channel'' [Ki nnally, K. W., Campo, M. L., & Tedeschi, H. T. (1989) J. Bioenerg. Bio membr. 21, 497-506] or ''megachannel'' [Petronilli, V., Szabo, I., & Z oratti, M. (1989) FEES Lett. 259, 137-143]. Therefore we conclude that the AAC, when converted into a large unselective channel, is a key co mponent in the MTP and thus is involved in the ischemia-reperfusion da mage and cytosolic Ca2+ oscillations. The channel opening in AAC is pr oposed to be caused by binding of Ca2+ to the cardiolipin, tightly bou nd to AAC, thus releasing positive charges within the AAC which open t he gate.