1. College of Pharmacy, Seoul National University, Seoul 151-742, Korea
2. Department of Biochemistry, College of Veterinary Medicine, Seoul National University, Seoul 151-742, Korea
3. Present address: Department of Biological Sciences, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea
4. Center for Food and Bioconvergence, College of Agriculture and Life Sciences, Seoul National University, Seoul 151-742, Korea