Distribution, biosynthesis and function of purine and pyridine alkaloids in Coffea arabica seedlings

Endogenous levels of purine and pyridine alkaloids were studied in different parts of 6-month-old Coffea arabica seedlings. In seedlings, caffeine was distributed mainly in leaves and cotyledons at concentrations varying from 43 to 104 mumol g(-1) dry weight. Essentially no caffeine was detected in roots or in older brown parts of shoots. In contrast, trigonelline was present in all parts of the seedlings. The concentration of trigonelline was highest in the upper part of the stems, including buds, which consist of young cells (180 mumol g(-1) dry weight), and was lowest in roots (25 mumol g(-1) dry weight). The trigonelline concentration in leaves was 60-80 mumol g(-1) dry weight, and the concentration in young leaves was higher than in older leaves. Purine alkaloid biosynthesis was estimated from the incorporation of radioactivity from [8-C-14]adenosine into purine alkaloids. Theobromine and caffeine were synthesized only in young leaves and young shoots including buds, but no biosynthetic activity was found in roots or aged cotyledons. Biosynthetic activity of trigonelline was estimated from the conversion of exogenously supplied [carboxyl-C-14] nicotinic acid to trigonelline. Trigonelline synthesis was found in all parts of the coffee seedlings. Metabolic fate studies indicated that large fractions of the radioactivity from [carboxyl-C-14]nicotinic acid (59% in leaves, 53% in cotyledons, 36% in stems and 29% in roots) were incorporated into trigonelline during a 4 h incubation period. Radioactivity was also found in NAD(P), NMN and nicotinamide. Only trace amounts of (CO2)-C-14, from [carboxyl-C-14] nicotinic acid were detected. These results suggest that caffeine accumulation is specific to above ground parts (leaves, cotyledons and shoots) of the seedlings and that biosynthesis is performed only in very young tissues, whereas trigonelline is distributed in all parts of coffee seedlings and biosynthetic activity is present even in mature parts. The differing roles of these two alkaloids are discussed. (C) 2003 Elsevier Ireland Ltd. All rights reserved.