1. Departments of Chemistry and Medicine, University of Alabama at Birmingham, Birmingham, Alabama 35294, Laboratory of Biological Chemistry, Division of Basic Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, The Burnham Institute, La Jolla Cancer Research Center, 10901 North Torrey Pines Road, La Jolla, California 92037, and Departments of Biochemistry and Toxicology, Southern Research Institute, 2000 Ninth Avenue South, Birmingham, Alabama 35205