N-Methyltransferases and 7-methyl - N9-Nucleoside jydrolase activity in Coffea arabica and the biosynthesis of caffeine
The incorporation of radioactivity from L-[14CH3]-methionine into caffeine by coffee frutis was enhanced by additions of theobromine and paraxanthine but was reduced by additions of theophylline and caffeine. Cell-free extracts prepared from seedlings, partially ripe and unripe coffee fruits showed that only the unripe green fruits contained significant methyltransferase and 7-methyl-N9-nucleoside hydrolase activity. The cell-free extracts catalysed the transfer of methyl groups from S-adenosyl-L-[14CH3]-methionine to 7-methylxanthine, and 7-methylxanthosine, producing theobromine and to theobromine producing caffeine. The two enzymic methylations exhibited a sharp pH max at 8.5 and a similar pattern of effects with metal chelators, thiol reagents and Mg2+ ions, which were slightly stimulating though not essential to enzyme activity. Paraxanthine (1,7-dimethylxanthine) was shown to be the most active among methylxanthines as methyl acceptors; however its formation from 1-methylxanthineand 7-methylxanthine was not detectable, and biosynthesis form paraxanthine in the intact plant would therefore appear not to occur. The apparent Km values are as folows: 7-methylxanthine 0.2 mM, theobromine 0.2 mM, paraxanthine 0.07 mM and S-adenosyl-L-methionine with each substrate 0.01 mM. The results suggest the pathway for caffeine biosynthesis in Coffea arabica is: 7-methylxanthosine - 7-methylxanthine - theobromine - caffeine
| Format: | biblioteca |
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| Published: |
1979
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| Subjects: | COFFEA ARABICA, BIOQUIMICA, BIOSINTESIS, CAFEINA, ACTIVIDAD ENZIMATICA, ANALISIS DE CAFEINA, |
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