1. Contribution from the Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K., Department of Analytical Sciences, SmithKline Beecham Pharmaceuticals, The Frythe, Welwyn, Hertfordshire AL6 9AR, U.K., and Department of Radiochemistry, SmithKline Beecham Pharmaceuticals, 709 Swedeland Road, King of Prussia, Pennsylvania 19406