Vacuolar localization of 1-sinapolglucose: L-malate sinapoyltransferase in protoplasts from cotyledons of Raphanus sativus.

The distribution of L-malate, sinapic acid esters and 1-sinapoylglucose: L-malate sinapoyltransferase (SMT) which catalyzes the synthesis of sinapoyl-L-malate were examined in preparations of protoplasts obtained from cotyledons of red radish (Raphanus sativus L. var. sativus). Vacuoles isolated from the protoplasts contained all of the SMT activity, all of the accumulated sinapic acid esters and about 50% of free L-malate present initially in the protoplasts. An esterase activity, acting on 1-sinapoyglucose, was found to be exclusively localized in the cytoplasm and a large proportion was found to be recoverable in a 100 000-g pellet obtained from protoplast lysates. The vacuoles were obtained after lysis of the protoplasts by osmotic shock and purification on a Ficoll gradient. The cytoplasmic contamination of vacuole preparations was found to be about 10%, as judged by enzymatic markers and microscopic inspection. No SMT activity was found in a 100 000-g pellet obtained from vacuole lysates. The results indicate that biosynthesis of sinapoyl-L-malate takes place within the central vacuoles of redradish cotyledons.