Purine and purine alkaloid metabolism in Camellia and Coffea plants

Abstract

The metabolism of purine nucleotides and purine alkaloids (e.g. caffeine and theobromine) are reviewed in tea and coffee plants. Purine metabolism in these plants is similar to that in other plants, which do not contain caffeine; however, tea and coffee plants have purine nucleotides, including those produced directly by the pathway of purine biosynthesis de novo, as effective precursors of caffeine. Xanthosine is the first methyl acceptor from S-adenosylmethionine in caffeine biosynthesis, and is also metabolized by a purine degradation pathway via xanthine. The regulation of purine alkaloid biosynthesis remains elusive, but the activity of the three N-methyltransferases is considered. The production and accumulation of the alkaloids are associated with the developmental stage of tissues (i.e. leaves, flowers, fruits and seeds) and with seasonal changes, especially in tea grown in temperate climates. The metabolism (especially biosynthesis) of purine alkaloids differs among Camellia species. In Coffea plants and in cultured cells, the rate of caffeine synthesis and turnover (i.e. biodegradation and/or biotransformation to xanthine or to methyluric acids) differs markedly among species. Ecological roles of the alkaloids have been reported, but their physiological significance in tea and coffee plants remains uncertain.