ON THE MECHANISM OF FATTY ACID-INDUCED PROTON TRANSPORT BY MITOCHONDRIAL UNCOUPLING PROTEIN

Uncoupling protein mediates electrophoretic transport of protons and a nions across the inner membrane of brown adipose tissue mitochondria, The mechanism and site of proton transport, the mechanism by which fat ty acids activate proton transport, and the relationship between fatty acids and anion transport are unknown, We used fluorescent probes to measure H+ and anion transport in vesicles reconstituted with purified uncoupling protein and carried out a comparative study of the effects of laurate and its close analogue, undecanesulfonate. Undecanesulfona te was transported by uncoupling protein with a K-m value similar to t hat observed for laurate as it activated H+ transport, Both laurate an d undecanesulfonate inhibited Cl- with competitive kinetics, Undecanes ulfonate inhibited laurate-induced H+ transport with competitive kinet ics, Undecanesulfonate and laurate differed in two important respects, (i) Laurate caused uncoupling protein-mediated H+ transport, whereas undecanesulfonate did not, (ii) Lauric acid was rapidly transported ac ross the bilayer by nonionic diffusion, whereas undecanesulfonic was n ot, We infer that the role of uncoupling protein in H+ transport is to transport fatty acid anions and that fatty acids induce H+ transport because they can diffuse electroneutrally across the membrane, Accordi ng to this hypothesis, uncoupling protein is a pure anion porter and d oes not transport protons; rather it is designed to enable fatty acids to behave as cycling protonophores.