REGULATION OF PURINE METABOLISM IN INTACT LEAVES OF COFFEA-ARABICA

The capacity of Coffea arabica leaves (5- x 5-mm pieces) to synthesize de novo and catabolize purine nucleotides to provide precursors for c affeine (1,3,7-trimethylxanthine) was investigated. Consistent with de novo synthesis, glycine, bicarbonate, and formate were incorporated i nto the purine ring of inosine 5'-monophosphate (IMP) and adenine nucl eotides (SIMGAAde); azaserine, a known inhibitor of purine de novo syn thesis, inhibited incorporation. Activity of the de novo pathway in C. arabica per g fresh weight of leaf tissue during a 3-h incubation per iod was 8 +/- 4 nmol of formate incorporated into IMP, 61 +/- 7 nmol i nto SIGMAAde, and 150 nmol into caffeine (the latter during a 7-h incu bation). Coffee leaves exhibited classical purine catabolism. Radiolab eled formate, inosine, adenosine, and adenine were incorporated into h ypoxanthine and xanthine, which were catabolized to allantoin and urea . Urease activity was demonstrated. Per g fresh weight, coffee leaf sq uares incorporated 90 +/- 22 nmol of xanthine into caffeine in 7 h but degraded 102 +/- 1 nmol of xanthine to allantoin in 3 h. Feedback con trol of de novo purine biosynthesis was contrasted in C. arabica and C ucurbita pepo, a species that does not synthesize purine alkaloids. En d-product inhibition was demonstrated to occur in both species but at different enzyme reactions.