An improved procedure for reconstitution of the uncoupling protein and in‐depth analysis of H+/OH− transport

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Abstract
An improved procedure for reincorporation of isolated uncoupling protein (UCP) from brown adipose tissue into phospholipid vesicles is reported and H+ uptake in K+‐deriven exchange diffusion quantitavely analyzed. UCP is isolated and reconstituted with medium‐length linear‐chain alkyl polyoxyethylene. In the critical step of vesicle formation, the stepwise removal of the detergent by polystyrene beads is applied. Vesicles are generated in the presence of solutes and buffers to be internalized which are then removed by gel filtration. The internal volume is about 4 μl/mg phospholipid with a vesicle diameter of 100 nm. One vescle contains, on average, six molecules UCP. The best results are obtained with purified egg yolk phosphatidylcholine. Addition of PtdEtn, PtdSer decreases the vesicle size and, still more, H+‐transport activity by UCP. Asolectin completely inactivates UCP. K+‐gradient‐driven H+ uptake is 80% inhibited by external GTP and 95% by internal plus external GTP. When H+ transport is recorded externally by a pH electrode and internally by pyranine, the kinetics show no delay resulting from intervening membrane‐bound H+ pools. Total H+ uptake after addition of carbonylcyanine m‐chlorophenylhydrazone (CCCP) and valinomycin corresponds to the diffusion between H+ and K+ and is unchanged by GTP. The linear correlation of H+‐transport inhibition to GTP binding demonstrates that all UCP molecules incorporated are equally active. The exchange diffusion between H+ uptake and K+ efflux is demonstrated using a K+ electrode and 86Rb measurements. Recording Ψ using 3,3′‐diispropylthiadicarbocyanine shows a rapid generation of Ψ on valinomycin addition, which decreases only slightly with H+ uptake, even after addition of CCCP or gramicidin. The Ψ collapses only after addition of external K+. By demonstrating that valinomycin‐induced K+ and H+ fluxes reflect relaxation into the diffusion equilibrium state, the transport rate of UCP can be evaluated as a first‐order rate, VH+/CH+, in which the rate, VH+, is related to H+‐uptake capacity, CH+. This allows quantitative comparison of transport rates independently of the variable CH+. The dependence on Ψ of H+ transport is measured by varying external K+ concentration. A virtually linear relation of the rate to the K+‐diffusion potential is observed, although the capacity is only slightly changed. The linear VH+/Ψ relationship resembles an open‐channel type of transport, but is discussed in terms of a low‐activation‐barrier type of carrier mechanism, in contrast to the log (VH+/Ψ) relation found for the ADP/ATP carrier with high activation barriers.