1. Department of Chemistry, Physical Chemistry I, and Biophysical Chemistry, University of Dortmund, Otto-Hahn Strasse 6, D-44227 Dortmund, Germany, Epithelial Cell Physiology, Max-Planck Institute of Molecular Physiology, Otto-Hahn Strasse 11, D-44202 Dortmund, Germany, and Instituto de Bioquímica Médica, Universidade Federal do Rio de Janeiro, Cidade Universitaria, Rio de Janeiro, Brazil