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A new caffeine biosynthetic pathway in tea leaves: utilisation of adenosine released from the S ‐adenosyl‐ L ‐methionine cycle
Author(s) -
Koshiishi Chie,
Kato Ayako,
Yama Sachiko,
Crozier Alan,
Ashihara Hiroshi
Publication year - 2001
Publication title -
febs letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.593
H-Index - 257
eISSN - 1873-3468
pISSN - 0014-5793
DOI - 10.1016/s0014-5793(01)02512-1
Subject(s) - caffeine , theobromine , adenosine kinase , adenosine , purine , biochemistry , chemistry , methionine , enzyme , biology , adenosine deaminase , amino acid , endocrinology
The four‐step caffeine biosynthetic pathway includes three methylation steps that utilise S ‐adenosyl‐ L ‐methionine (SAM) as the methyl donor. In the process SAM is converted to S ‐adenosyl‐ L ‐homocysteine (SAH) which in turn is hydrolysed to L ‐homocysteine and adenosine. Significant amounts of radioactivity from [methyl‐ 14 C]methionine and [methyl‐ 14 C]SAM were incorporated into theobromine and caffeine in young tea leaf segments, and very high SAH hydrolase activity was found in cell‐free extracts from young tea leaves. Substantial amounts of radioactivity from [adenosyl‐ 14 C]SAH were also recovered as theobromine and caffeine in tea leaf segments, indicating that adenosine derived from SAH is utilised for the synthesis of the purine ring of caffeine. From the profiles of activity of related enzymes in tea leaf extracts, it is proposed that the major route from SAM to caffeine is a SAM→SAH→adenosine→adenine→AMP→IMP→XMP→xanthosine→7‐methylxanthosine→7‐methylxanthine→theobromine→caffeine pathway. In addition, direct adenosine kinase‐catalysed formation of AMP from adenosine may participate as an alternative minor route. The activity of two of the three N ‐methyltransferase activities involved in caffeine biosynthesis and part of the activities of SAH hydrolase, adenosine nucleosidase, adenine phosphoribosyltransferase and adenosine kinase were located in tea chloroplasts. In contrast, no detectable activity of SAM synthetase was associated with the purified chloroplast fraction. This is a first demonstration that the purine skeleton of caffeine is synthesised from adenosine released from the SAM cycle.