Finding of UDP-Glucose: Nicotinic Acid-N-glucosyltransferase Activity in Cultured Tobacco Cells and Its Properties

We have already identified N-(β-d-glucopyranosyl)nicotinic acid, which was isolated from cultured tobacco cells, as a novel niacin metabolite. The enzyme activity for the biosynthesis of this compound, UDP-glucose: nicotinic acid-N-glucosyltransferase, was found in cell-free extracts from cultured cells of tobacco (Nicotiana tabacum XD-6) and properties were investigated in this experiment. The general catalytic properties were as follows: optimum temperature was 25°C, optimum pH was 6.1, the K m for NiA and UDP-glucose were calculated at 220 μμ and 4.3 mm, respectively. The enzyme activity was lowered by heavy metal ions, isonicotinic acid, benzoic acid, and 6-hydroxynicotinic acid. The roles of niacin metabolites in cultured tobacco cells are discussed in relation to the enzyme.