1. Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113, Japan, Department of Biomolecular Science, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274, Japan, R & D Group of Pharmaceuticals, Central Research Institute, Ishihara Sangyo Kaisha, Ltd., 2-3-1 Nishi-shibukawa, Kusatsu, Shiga 525, Japan