BIOSYNTHESIS OF CAFFEINE IN LEAVES OF COFFEE

The levels of endogenous caffeine and theobromine were much higher in buds and young leaves of Coffea arabica L. cv Kent than in fully devel oped leaves. Biosynthesis of caffeine from C-14-labeled adenine, guani ne, xanthosine, and theobromine was observed, whereas other studies (H . Ashihara, A.M. Monteiro, T. Moritz, F.M. Gillies, A. Crozier [1996] Planta 198: 334-339) have indicated that there is no detectable incorp oration of label into caffeine when theophylline and xanthine are used as substrates for in vivo feeds with leaves of C. arabica. The capaci ty for caffeine biosynthesis, especially from guanine and xanthosine, was reduced markedly in both fully developed mature and aged leaves. D ata obtained in pulse-chase experiments with young leaves indicate the operation of an AMP --> IMP --> xanthosine 5'-monophosphate (or CMP - -> guanosine) --> xanthosine --> 7-methylxanthosine --> 7-methylxanthi ne --> theobromine --> caffeine pathway. The data obtained provide str ong evidence against proposals by G.M. Nazario and C.J. Lovatt ([1993] Plant Physiol 103: 1203-1210) concerning the independence of caffeine and theobromine biosynthesis pathways and the role of xanthine as a k ey intermediate in caffeine biosynthesis.