Enzymatic Synthesis of 1-Sinapoylglucose from Free Sinapic Acid and UDP-Glucose by a Cell-Free System from Raphanus sativus Seedlings

Raphanus, Brassicaceae, Phenylpropanoid Metabolism, Glucose Ester, Esterification. Protein extracts from seedlings of Raphanus sativus catalyze the transfer of the glucosyl moiety of UDP-glucose to the carboxyl group of phenolic acids. Enzymatic activity was determined spectrophotometrically by measuring the increase in absorbance at 360 nm and/or by the aid of high performance liquid chromatography (HPLC). From 12 phenolic acids tested as acceptors, sinapic acid was by far the best substrate. The glucosyltransfer to sinapic acid has a pH optimum near 7 and requires as SH group for activity, pChloromercuribenzoate (PCMB) inhibits activity, which can be restored by the addition of dithiothreitol (DTT). The formation of 1-sinapoylglucose was found to be a reversible reaction, sin­ ce the addition of UDP results in a breakdown of the ester.