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, and WU/TNO Center for Food Toxicology, P.O. Box 8000, 6700 EA Wageningen, The Netherlands