1. Natural Products Research Institute, Seoul National University, 28 Yungun, Jongro, Seoul 110-460, Korea, College of Pharmacy, Seoul National University, San 56-1, Shilim, Kwanak, Seoul 151-742, Korea, and Korea Research Institute of Bioscience and Biotechnology P.O. Box 115, Yusong, Taejon 305-600, Korea pennkim@snu.ac.kr