New substrates and competitive inhibitors of the Cl- translocating pathway of the uncoupling protein of brown adipose tissue mitochondria

A large number of new substrates for anion uniport by the uncoupling protein of brown adipose tissue mitochondria have been found. These include alkylsulfonates, alkylsulfates and their derivatives, benzenesulfonate, oxohalogenides, hypophosphate, hexafluorophosphate, and pyruvate. Although the spectrum of anion selectivity is far wider than had previously been suspected, there are strong structural requirements for transport. The anion must be monovalent, and polar groups must not be attached to alkyl or aryl chains. The most striking finding is that transport increases dramatically with anion hydrophobicity. Anions that are transported are shown to compete with Cl- for transport by the reconstituted uncoupling protein. For each anion, the Ki for GDP inhibition of transport increases with its rate of transport and correlates inversely with its Ki for competitive inhibition of Cl- transport. For alkylsulfonates, transport rate, Ki for GDP inhibition, and Ki for inhibition of Cl- transport each depend monotonically on alkyl chain length. These findings suggest several new hypotheses relating to the molecular mechanism of transport through uncoupling protein and suggest explanations for observed functional differences among porters belonging to the same gene family.