ELECTRICAL CURRENTS ASSOCIATED WITH NUCLEOTIDE TRANSPORT BY THE RECONSTITUTED MITOCHONDRIAL ADP ATP CARRIER/

The electrophoretic export of ATP against the import of ADP in mitocho ndria bridges the intra- versus extramitochondrial ATP potential gap, Here we report that the electrical nature of the ADP/ATP exchange by t he mitochondrial ADP/ATP carrier (AAC) can be directly studied by meas uring the electrical currents via capacitive coupling of AAC-containin g vesicles on a planar lipid membrane. The currents were induced by th e rapid liberation of ATP or ADP with UV flash photolysis from caged n ucleotides, Six different transport modes of the AAC were studied: het eroexchange with either ADP or ATP inside the vesicles, initiated by p hotolysis of caged ATP or ADP; homoexchange with ADP(ex)/ADP(in) or AT P(ex)/ATP(in); and caged ADP or ATP with unloaded vesicles. The hetero exchange produced the largest currents with the longest duration in li ne with the electrical charge difference ATP(4-) versus ADP(3-), Surpr isingly, also in the homoexchange and with unloaded vesicles, small cu rrents were measured with shorter duration, In all three modes with ca ged ATP, a negative charge moved into the vesicles and with caged ADP it moved out of the vesicles, All currents were completely inhibited b y a mixture of the inhibitors of the AAC, carboxyatractyloside and bon gkrekate, which proves that the currents are exclusively due to AAC fu nction, The observed charge movements in the heteroexchange system agr ee with the prediction from transport studies in mitochondria and reco nstituted vesicles, The unexpected charge movements in the homoexchang e or unloaded systems are interpreted to reveal transmembrane rearrang ements of charged sites in the AAC when occupied with ADP or ATP, The results also indicate that not only ATP(4-) but also ADP(3-) contribut e, albeit in opposite direction, to the electrical nature of the ADP/A TP exchange, which is at variance with former conclusions from biochem ical transport studies, These measurements open up new avenues of stud ying the electrical interactions of ADP and ATP with the AAC.