Pyridine nucleotide cycle and trigonelline (N‐methylnicotinic acid) synthesis in developing leaves and fruits of Coffea arabica

We examined the biosynthesis of trigonelline in leaves and fruits of Arabica coffee ( Coffea arabica ) plants. [ 3 H]Quinolinic acid, which is an intermediate of de novo pyridine nucleotide synthesis, and [ 14 C]nicotinamide and [ 14 C]nicotinic acid, which are degradation products of NAD, were converted to trigonelline and pyridine nucleotides. These tracer experiments suggest that the pyridine nucleotide cycle, nicotinamide → nicotinic acid → nicotinic acid mononucleotide (NaMN) → nicotinic acid adenine dinucleotide (NaAD) → NAD → nicotinamide mononucleotide (NMN) → nicotinamide, operates in coffee plants, and trigonelline is synthesized from nicotinic acid formed in the cycle. Trigonelline accumulated up to 18 µmol per leaf in developed young leaves, and then decreased with age. Although the biosynthetic activity of trigonelline from exogenously supplied [ 14 C]nicotinamide was observed in aged leaves, the endogenous supply of nicotinamide may be limited, reducing the contents in these leaves. Trigonelline is synthesized and accumulated in fruits during development. The trigonelline synthesis in pericarps is much higher than that in seeds, but its content in seeds is higher than pericaps, so that some of the trigonelline synthesized in the pericarps may be transported to seeds. Trigonelline in seeds may be utilized during germination, as its content decreases. Trigonelline synthesis from [ 14 C]nicotinamide was also found in Theobroma cacao plants, but instead of trigonelline, nicotinic acid‐glucoside was synthesized from [ 14 C]nicotinamide in Camellia sinensis plants.