1. Laboratory for Physiological Chemistry, University Medical Center; 3508 AB Utrecht The Netherlands
2. Bijvoet Center for Biomolecular Research, Utrecht University; 3584 CH Utrecht The Netherlands
3. Present address: Biomedical Research Laboratories, Sankyo Co. Ltd, 2-58, Hiromachi 1-chome Shinagawa-ku; Tokyo 140-8710 Japan
4. Present address: Department of Gynaecology, University Hospital Groningen; 9700 RB Groningen The Netherlands
5. Department of Medical Biochemistry, CMU, University of Geneva; 1211 Geneva 4 Switzerland