1. From the Department of Internal Medicine and Pharmacology (K.G.L., D.W.N., F.M.F.), University of Iowa, Roy J. and Lucille A. Carver School of Medicine and the Veterans Administration Medical Center (K.G.L., D.W.N.), Iowa City, IA; and the Laboratory of Bioorganic Chemistry (J.W., Y.C.), National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD.