1. Department of Physiology and Pharmacology, Texas A&M University, TVMC, College Station, Texas 77843-4466, Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755-1812, and Department of Pathobiology, Texas A&M University, College Station, Texas 77843-4467