N-Methyltransferase catalyzing the transfer of methyl groups from [14CH3]SAM to xanthosine, producing 7-methylxanthosine, was extracted in cell-free systems from tea leaves. The purine nucleotides, nucleosides, and bases, except for xanthosine, were all inactive substrates in the methylation at the N-7 position of their purine rings. This confirmed that the methylation of xanthosine to 7-methylxanthosine is the first step of methylation in the pathway for caffeine biosynthesis. The optimum pH for the methyltransferase is 7.5 to 8.0. PCMB (0.5 mM), Zn2+ (1 HIM) and Cu2+ (ImM) strongly inhibited the enzyme activity. The Km values for xanthosine and SAM are 0.25 HIM at 3.9μM of SAM and 3.3μM at 0.40 HIM of xanthosine, respectively.
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Negishi et al. (1985) studied this question.