1. Department of Membrane Cell Biology, University of Groningen, 9700 AD Groningen, The Netherlands, Department of Pharmacy, Martin-Luther-University Halle, Wolfgang-Langenbeck-Strasse 4, D 06120 Halle, Germany, Department of Chemistry, University Konstanz, Box M725, D 78457 Konstanz, Germany, and Institute of Physiological Chemistry, Martin-Luther-University Halle, Hollystrasse 1, D 06097 Halle, Germany