SEPARATE DE-NOVO AND SALVAGE PURINE POOLS ARE INVOLVED IN THE BIOSYNTHESIS OF THEOBROMINE BUT NOT CAFFEINE IN LEAVES OF COFFEA-ARABICA L

In Coffea arabica leaves, the purine ring of theobromine (3,7-dimethyl xanthine) and caffeine (1,3,7-trimethylxanthine) is provided by de nov o purine biosynthesis: (a) [C-14]glycine, [C-14]bicarbonate, and [C-14 ]formate were incorporated into inosine 5'-monophosphate (IMP), sum of adenine nucleotides (SIGMAAde), theobromine, and caffeine; and (b) in corporation of [C-14]formate into IMP, SIGMAAde, theobromine, and caff eine was inhibited by azaserine, a known inhibitor of de novo purine b iosynthesis. Capacity of coffee leaves to salvage added purines was de monstrated by incorporation of [C-14]hypoxanthine into SIGMAAde and th e incorporation of [C-14]adenosine, [C-14]adenine, [C-14]inosine, and [C-14]hypoxanthine into both theobromine and caffeine. Consistent with synthesis of theobromine from two separate purine nucleotide pools, o ne synthesized de novo and one via salvage, added xanthine 5'-monophos phate (XMP), inosine, or hypoxanthine failed to reduce the incorporati on of [C-14]formate into theobromine but diluted the specific radioact ivity of [C-14]adenosine and [C-14]adenine incorporated into theobromi ne. Evidence that theobromine is not the immediate precursor of caffei ne is provided: (a) [C-14]xanthine was incorporated into caffeine but not into theobromine; (b) exogenous xanthine diluted the specific radi oactivity of caffeine synthesized from [C-14]adenine and [C-14]hypoxan thine but caused accumulation of radiolabel in theobromine; (c) allopu rinol, a known inhibitor of the conversion of hypoxanthine to xanthine , reduced incorporation of [C-14]adenine and [C-14]hypoxanthine into c affeine but caused accumulation of radiolabel in theobromine; and (d) incorporation of [C-14]formate into caffeine, but not into theobromine , was reduced by added XMP, inosine, or hypoxanthine.