1. Institute of Biochemistry and Molecular Biology II, Medical Faculty, Heinrich-Heine University, Düsseldorf, Germany;
2. Institute of Biochemistry, Christian-Albrechts-University, Kiel, Germany;
3. Institute of Medical Microbiology and Hospital Hygiene, Heinrich-Heine University, Düsseldorf, Germany
4. Present address: Institute of Biochemistry, Christian-Albrechts-University, Kiel, Germany