TRANSPORT OF ANIONS AND PROTONS BY THE MITOCHONDRIAL UNCOUPLING PROTEIN AND ITS REGULATION BY NUCLEOTIDES AND FATTY-ACIDS - A NEW LOOK AT OLD HYPOTHESES

The uncoupling protein generates heat by catalyzing electrophoretic pr oton transport across the inner membrane of brown adipose tissue mitoc hondria. It also transports Cl- and other monovalent anions, and both proton and anion transport are inhibited by purine nucleotides. Severa l long standing hypotheses bear on specific aspects of Cl- transport, H+ transport, and nucleotide gating mechanisms in uncoupling protein. We reevaluated these hypotheses in mitochondria and liposomes reconsti tuted with purified uncoupling protein; GDP inhibition is strictly non competitive with Cl- and unaffected by either transmembrane electrical potential or fatty acids. The K-m and V-max values for Cl- are indepe ndent of pH, arguing against a common binding site for Cl- and OH- ion s. Cl- transport was inhibited by fatty acids and stimulated by fatty acid removal, refuting the consensus hypothesis that there is no inter action between fatty acids and anion transport through uncoupling prot ein. These results support a mechanism in which the transport pathway for anions is identical with the fatty acid binding site and distinct from the nucleotide binding site.