1. Department of Medical Biochemistry and Genetics, Biochemistry Laboratory A, The Panum Institute, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen N, Denmark, Geological Institute, University of Copenhagen, Østervoldgade 10, DK-1350 Copenhagen K, Denmark, and Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark