1. Department of Analytical Chemistry, University of Lund, Box 124, S-221 00 Lund, Sweden, Department of Analytical Chemistry, Faculty of Pharmacy, University of Alcalá de Henares, 28 871 Alcalá de Henares, Madrid, Spain, and Enzyme Chemistry Laboratory, Institute of Biochemistry, Mokslininku 12, 2600 Vilnius, Lithuania