1. School of Biomedical Sciences, University of Ulster, Newtownabbey, Co. Antrim BT37 0QB, N. Ireland, U.K.
2. CRC Gray Laboratory, P.O. Box 100, Mount Vernon Hospital, Northwood, Middx. HA6 2JR, U.K.
3. Department of Biology and Biochemistry, Brunel University, Uxbridge, Middx. UB8 3PH, U.K.