Development of 1-O-sinapoyl-β-D-glucose: L-malate sinapoyltransferase activity in cotyledons of red radish (Raphanus sativus L. var. sativus).

Protein preparations from cotyledons of red radish (Raphanus sativus L. var. sativus) catalyzed the the formation of depsides between cinnamic acids and L-malate, using 1-O-acyl glucose conjugates as the donors. This activity showed an absolute acceptor specificity towards L-malate and a pronounced donor specificity with 1-sinapoylglucose (1-O-sinapoyl-β-D-glucose). Maximal rate of sinapoyl-L-malate formation was found to be at pH 6.3, and there was no requirement for metal ions or sulfhydryl group reagents. The K m values were found to be 0.46 mM for 1-sinapoylglucose and 54 mM for L-malate. Protein extracts obtained from seedlings at different stages of seedling development did not significantly differ with respect to the properties of the enzymatic activity. Appearance and development of extractable activities correlated well with the in vivo transacylation kinetics of 1-sinapoylglucose to sinapoyl-L-malate during seedling growth. Maximal activity was extracted from 10-14-d-old seedlings and found to be at 67 pkat pair(-1) of cotyledons. This new enzymatic activity in phenylpropanoid metabolism refers to an enzyme which can be classified as 1-sinapoylglucose: L-malate sinapoyltransferase (SMT) (EC 2.3.1.-).