1. 1Department of Biochemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.
2. 2Department of Biochemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.
3. 3Department of Biochemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.
4. 4Department of Biochemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.