1. Glynn Laboratory of Bioenergetics, Department of Biology, University College London, Gower Street, London WC1E 6BT, U.K., Medical Research Council Dunn Human Nutrition Unit, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 2XY, U.K., and The Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania, 1004 Stellar-Chance Building, 422 Curie Boulevard, Philadelphia, Pennsylvania 19104