1. Division of Toxicology, Wageningen University, Tuinlaan 5, 6703 HE, Wageningen, The Netherlands, Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA, Wageningen, The Netherlands, TNO Nutrition and Food Research, P.O. Box 360, 3700 AJ, Zeist, The Netherlands, WU/TNO Centre for Food Toxicology, P.O. Box 8000, 6700 EA, Wageningen, The Netherlands, and N.V. Organon, P.O. Box 20, 5340 BH, Oss, The Netherlands