1. National Institute of Bioscience and Human Technology, 1--1 Higashi, Tsukuba, Ibaraki 305--8566, Japan
2. Biological Laboratory, Nippon Medical School, Kawasaki 211--0063, Japan
3. School of Dentistry, Hokkaido University, Sapporo 060, Japan
4. Institute of Low Temperature Science, Hokkaido University, Sapporo 060, Japan