1. Laboratory of Chemistry, National Heart, Lung, and Blood Institute, Bethesda, MD 20892.
2. Institute of Reproductive Biology, University of Texas, Austin, TX 78712.
3. Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD 20892.
4. Department of Chemistry, University of Texas, Austin, TX 78712.