1. Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060, Japan, Institute for Life Science Research, Asahi Chemical Industry Co., Ltd., Mifuku, Ohito-cho, Shizuoka 410-23, Japan, and Rega Institute for Medical Research, Katholieke Universiteit Leuven, B-3000 Leuven, Belgium