¹⁴C-labelled methionine, xanthosine, and 7-methylxanthosine were given to excised tea shoots. The methyl group of methionine was incorporated into 7-methylxanthosine (ca. 10%) in the earlier period of incubation after the uptake. About 50% of the radioactivity of xanthosine was rapidly incorporated into caffeine via 7-methylxanthosine, 7-methylxanthine, and theobromine within 24 hr. 7-Methylxanthosine was also converted into caffeine at a high rate. The results suggest that the pathway for caffeine biosynthesis is as follows: xanthosine → 7-methylxanthosine → 7-methylxanthine → theobromine → caffeine.

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