Uncoupling protein mediates electrophoretic transport
of protons and anions across the inner membrane of brown adipose tissue mitochondria.
The mechanism and site of proton transport, the mechanism by which fatty 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. Undecanesulfonate was transported by uncoupling protein
with a Km value similar to that observed for laurate as it activated H+ transport.
Both laurate and undecanesulfonate inhibited Cl- with competitive kinetics.
Undecanesulfonate inhibited laurate-induced H+ transport with competitive
kinetics. 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 across the bilayer by nonionic
diffusion, whereas undecanesulfonic was not. 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. According to this hypothesis, uncoupling protein is a pure anion
porter and does not transport protons; rather it is designed to enable fatty
acids to behave as cycling protonophores.
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