1. Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146, Department of Drug Metabolism and Safety Assessment, Schering-Plough Research Institute, Kenilworth, New Jersey 07033-1300, and Physiology and Pharmacology, School of Biomedical Sciences, The University of Queensland, St. Lucia, Brisbane, Queensland, Australia 4072