PLANT PHYSIOLOGY , Vol 103, Issue 4 1203-1210, Copyright © 1993 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Separate de Novo and Salvage Purine Pools Are Involved in the Biosynthesis of Theobromine but Not Caffeine in Leaves of Coffea arabica L
G. M. Nazario and C. J. Lovatt
Department of Botany and Plant Sciences, University of California, Riverside, California 92521-0124
In Coffea arabica leaves, the purine ring of theobromine
(3,7-dimethylxanthine) and caffeine (1,3,7-trimethylxanthine) is provided
by de novo purine biosynthesis: (a) [14C]glycine, [14C]bicarbonate, and
[14C]formate were incorporated into inosine 5[prime]- monophosphate (IMP),
sum of adenine nucleotides ([sigma]Ade), theobromine, and caffeine; and (b)
incorporation of [14C]formate into IMP, [sigma]Ade, theobromine, and
caffeine was inhibited by azaserine, a known inhibitor of de novo purine
biosynthesis. Capacity of coffee leaves to salvage added purines was
demonstrated by incorporation of [14C]hypoxanthine into [sigma]Ade and the
incorporation of [14C]adenosine, [14C]adenine, [14C]inosine, and
[14C]hypoxanthine into both theobromine and caffeine. Consistent with
synthesis of theobromine from two separate purine nucleotide pools, one
synthesized de novo and one via salvage, added xanthine
5[prime]-monophosphate (XMP), inosine, or hypoxanthine failed to reduce the
incorporation of [14C]formate into theobromine but diluted the specific
radioactivity of [14C]adenosine and [14C]adenine incorporated into
theobromine. Evidence that theobromine is not the immediate precursor of
caffeine is provided: (a) [14C]xanthine was incorporated into caffeine but
not into theobromine; (b) exogenous xanthine diluted the specific
radioactivity of caffeine synthesized from [14C]adenine and
[14C]hypoxanthine but caused accumulation of radiolabel in theobromine; (c)
allopurinol, a known inhibitor of the conversion of hypoxanthine to
xanthine, reduced incorporation of [14C]adenine and [14C]hypoxanthine into
caffeine but caused accumulation of radiolabel in theobromine; and (d)
incorporation of [14C]formate into caffeine, but not into theobromine, was
reduced by added XMP, inosine, or hypoxanthine.
