N-methyl nucleosidase from tea leaves.

N-Methyl nucleoside hydrolase (TV-methyl nucleosidase, N-MeNase), which hydrolyzes 7-methylxanthosine to produce 7-methylxanthine, was detected in tea-leaf extracts and separated from adenosine nucleosidase (ANase, EC 3.2.2.7) by DEAE-cellulose column chromatography. The optimum pH for the N-MeNase ranged from 8.0 to 8.5. The enzyme was strongly inhibited by EDTA. Inhibition by the hydrolysis products of 7-methylxanthosine and 7-methylinosine was also observed. The molecular weight was estimated to be about 55, 000 by gel-filtration. Among purine and N-methylpurine nucleosides, 3- and 7-methylpurine nucleosides were hydrolyzed preferentially by N-MeNase. On the other hand, ANase could not hydrolyze 7-methylxanthosine, although the enzyme showed high activity toward 7-methyladenosine. As a result, it is suggested that N-MeNase catalyzes the hydrolysis reaction of 7-methylxanthosine in the pathway of caffeine biosynthesis, whereas ANase is not directly concerned with it.