1. Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway; Department of Chemistry, Biotechnology and Food Science, Agricultural University of Norway, P.O. Box 5003, N-1432 Aas, Norway; and Animal Health Division, ALPHARMA, P.O. Box 158 Skøyen, N-0212 Oslo, Norway