Fatty acid (FA) uniport via mitochondrial uncoupling protein (UcP) was detected fluorometrically with PBFI, potassium-binding benzofuran phthalate and SPQ, 6-methoxy-N-(3-sulfopropyl)-quinolinium, indicating K+ and H+, respectively. The FA structural patterns required for FA flip-flop, UcP-mediated FA uniport, activation of UcP-mediated H+ transport in proteoliposomes, and inhibition of UcP-mediated Cl- uniport by FA, were identical. Positive responses were found exclusively with FA which were able to flip-flop in a protonated form across the membrane and no responses were found with 'inactive' FA lacking the flip-flop ability. The findings support the existence of FA cycling mechanism.
Ježek, P., Modrianský, M., & Garlid, K. D. (1997). A structure-activity study of fatty acid interaction with mitochondrial uncoupling protein. FEBS Letters, 408(2), 166–170. https://doi.org/10.1016/S0014-5793(97)00335-9