1. Contribution from the Salk Institute for Biological Studies, P.O. Box 85800, San Diego, California 92186, Department of Biochemistry, Oxford University, South Parks Road, Oxford OX1 3QU, U.K., and Laboratory of Medical Chemistry, Rega Institute, Katholieke Universiteit Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium