|Budget Amount *help
¥7,800,000 (Direct Cost : ¥7,800,000)
Fiscal Year 1997 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1996 : ¥7,000,000 (Direct Cost : ¥7,000,000)
Several purse-chase experiments using various ^<14>C-and ^<15>N-Precursors indicate the operation of an IMP*XMP*xanthosine*7-methylxanthosine*7-methylxanthine (7mX) *theobromine* caffeine pathway. In young tea leaves, two N-methyltransferases (NMTs) may participate in the conversion of xanthosine to caffeine. The latter NMT activity was found to be associated with a purified chloroplast preparation obtained by using a discontinuous Percoll density gradient. This the second NMT was highly purified. IMP utilized for caffeine synthesis is supplied by the purine nucleotide biosynthesis de novo. The alternative routes of caffeine biosynthesis, such as the AMP and the GMP pathways are also proposed. Purine alkaloid catabolism pathways in young, mature and aged leaves of tea and coffee were investigated by incubating leaf sections with ^<14>C-labelled theobromine, caffeine, theophylline and xanthine. The data obtained demonstrate that theobromine is the immediate precursor of caffeine, which accumulates in tea leaves because its conversion to theophylline is the rate limiting step in the purine alkaloid catabolism pathway. The main fate of [8-^<14>C] theophylline incubated with mature and aged tea and coffee leaves, and to a lesser extent young leaves, is conversion to 3-methylxanthine and onto xanthine which is degraded to ^<14>CO_2 via the purine catabolism pathway. However, with young tea leaves, sizable amounts of [8-^<14>C] theophylline were salvaged for the synthesis of caffeine via a 3-methylxanthine*theobromine*caffeine pathway. Trace amounts of [2-^<14>C] xanthine were also salvaged for caffeine biosynthesis in young tea leaves, by conversion to 3-methylxanthine. Feeds of [2-^<14>C] xanthine to young leaves also indicated that 3-methylxanthine, as well as being salvaged for theobromine and caffeine production, is also converted, via N-1-methylation, to theophylline.