Changes in trigonelline (N-methylnicotinic acid) content and nicotinic acid metabolism during germination of mungbean (Phaseolus aureus) seeds

Changes in trigonelline content and in biosynthetic activity were determined in the cotyledons and embryonic axes of etiolated mungbean (Phaseolus aureus) seedlings during germination. Accumulation of trigonelline (c. 240 nmol per pair of cotyledons) was observed in the cotyledons of dry seeds; trigonelline content decreased 2 d after imbibition. Trigonelline content in the embryonic axes increased with seedling growth and reached a peak (c. 380 nmol per embryonic axis) at day 5. Trigonelline content did not change significantly during the differentiation of hypocotyls, and the concentration was greatest in the apical 5 mm. Nicotinic acid and nicotinamide were better precursors for pyridine nucleotide synthesis than quinolinic acid, but no great differences were found in the synthesis of trigonelline from these three precursors. Trigonelline synthesis was always higher in embryonic axes than in cotyledons. Activity of quinolinate phosphoribosyltransferase (EC 2.4.2.19), nicotinate phosphoribosyltransferase (EC 2.4.2.11), and nicotinamidase (EC 3.5.1.19) was found in cotyledons and embryonic axes, but no nicotinamide phosphoribosyltransferase (EC 2.4.2.12) activity was detected. It follows that quinolinic acid and nicotinic acid were directly converted to nicotinic acid mononucleotide by the respective phosphoribosyltransferases, but nicotinamide appeared to be converted to nicotinic acid mononucleotide after conversion to nicotinic acid. Trigonelline synthase (nicotinate N-methyltransferase, EC 2.1.1.7) activity increased in the embryonic axes, but decreased in cotyledons during germination. 14C]Nicotinic acid and trigonelline absorbed by the cotyledons were transported to the embryonic axes during germination. Trigonelline had no effect on the growth of seedlings, but nicotinic acid and nicotinamide significantly inhibited the growth of roots. Based on these findings, the role of trigonelline synthesis in mungbean seedlings is discussed.