1. Department of Biochemistry and Molecular Biology, Georgetown University School of Medicine, NW, Washington, DC 20007, Institut fur Physiologie Chemie, Universitats-Krankenhaus, Eppendorf, 2000 Hamburg 20, West Germany, Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, and Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore, Maryland 21201, and Bio-Rad Laboratories, 2000 Alfred Nobel Drive, Hercules, California 94547