1. Department of Research and Development, Statens Serum Institut, Artillerivej 5, DK-2300 Copenhagen, Denmark, Department of Chemistry, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark, and BioCentrum-DTU, Biochemistry and Nutrition, Technical University of Denmark, DK-2800 Lyngby, Denmark