1. 1Department of Biological Chemistry, British Industrial Biological Research Association, Woodmansterne Road, Carlshalton, Surrey SM5 4DS, U.K.
2. 2Department of Biological Chemistry, British Industrial Biological Research Association, Woodmansterne Road, Carlshalton, Surrey SM5 4DS, U.K.
3. 3Department of Biological Chemistry, British Industrial Biological Research Association, Woodmansterne Road, Carlshalton, Surrey SM5 4DS, U.K.