1. Laboratory of Biochemistry, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands, Department of Tanning Materials & Proteins, National Research Centre, 12622 Dokki, Cairo, Egypt, Division of Toxicology, Wageningen University, Tuinlaan 5, 6703 HE Wageningen, The Netherlands, and TNO/WU Centre for Food Toxicology, P.O. Box 8000, 6700 EA Wageningen, The Netherlands