A protein fraction of mitochondria from human acute lymphocytic leukemia cells, which could be reconstituted into proteoliposomes to have dCTP transport activity, has been partially purified by hydroxyapatite and blue Sepharose chromatography. The dCTP transport activity in proteoliposomes was time-dependent and could be activated by Ca 2؉ and to a lesser extent by Mg 2؉ . None of the other divalent cations tested could activate the transport activity. The K m value of dCTP in the presence of

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... 2؉ was shown to be 3 M. dCDP but not dCMP or dCyd could inhibit the transport activity. Other deoxynucleoside triphosphates could also inhibit the uptake of dCTP with the potency dGTP ‫؍‬ dATP > TTP. Although ATP could competitively inhibit dCTP uptake with a K i value of 8 M, the reconstituted dCTP uptake activity was not sensitive to the ATP/ADP carrier inhibitor atractyloside or the sulfhydryl reagent N-ethylmaleimide. This suggests that the dCTP transport system studied is not the same as the ATP/ADP carrier. In conclusion, these studies describe the first functionally reconstituted mitochondrial carrier that displays an efficient transport activity for dCTP.