The reconstituted ADP/ATP carrier can mediate H+ transport by free fatty acids, which is further stimulated by mersalyl.

In a reconstituted system, the participation of the ATP/ADP carrier (AAC) in the free fatty acid (FFA)-induced proton transport was demonstrated (i) by direct measuring of the proton transport through the membranes of AAC proteoliposomes and (ii) by monitoring of the transmembrane potential delta psi in AAC-cytochrome-c oxidase (COX)-coreconstituted proteoliposomes. FFA increased the initial rate of proton transport in AAC proteoliposomes and decreased delta psi in AAC-COX proteoliposomes. Inhibitors of AAC suppressed the effects of FFA. Without AAC or with inactive AAC, FFA cannot maintain proton leakage through the membrane. In these cases, even a small increase of delta psi was induced by FFA. These results demonstrate for the first time with purified components a participation of AAC in FFA-induced proton transport supporting an earlier suggestion (Skulachev, V.P. (1991) FEBS Lett. 294, 158-162). Mersalyl treatment of the AAC-COX proteoliposomes resulted in an increase of the AAC-mediated protonophoric action of FFA. Mersalyl also sensitized the protonophoric action of the FFA against nucleotides so that even guanine nucleotides, which are inactive in transport, become inhibitory. The effect of mersalyl is rationalized in terms of a specific interaction with cysteine 159 being attracted as anion by surrounding positive charges. This might open a gate similarly as suggested for eosin 5-maleimide interaction (Majima, E., Koike, H., Hong, Y.-M., Shinohara, Y., and Terada, H. (1993) J. Biol. Chem. 268, 22181-22187) and, thus, transform the AAC into undirectional transport mode.