CARDENOLIDE GLUCOSYLTRANSFERASES AND GLUCOHYDROLASES IN LEAVES AND CELL-CULTURES OF 3 DIGITALIS (SCROPHULARIACEAE) SPECIES

The enzymatic synthesis and hydrolysis of primary cardiac glycosides in leaves and suspension cultures of Digitalis lanata, D. purpurea and D. heywoodii were investigated. Cardenolide glucosyltransferase, with an activity ranging from 0.1 to 21.5µkat/kg protein, was demonstrated in leaves, callus and suspension-cultured cells of all three species examined. Cardenolide-hydrolysing glucosidases (digilanidases) were solubilized from leaves of the Digitalis species using detergent-containing buffers. The specific enzyme activities in buffered leaf extracts reached 1 to 5 mkat/kg. The properties of the cardenolide glucohydrolases from Digitalis lanata, D. heywoodii, and D. purpurea were compared. Optimal enzyme activities occurred at around pH 4.5 and 50 – 60°C. Considerable variations in substrate preferences were observed among the cardenolide glucosidases of the three species. A method was devised to determine cardenolide glucosyltransferase in tissues with high digilanidase activity. Extraction with detergent-free buffers containing 10mM D-gluconic acid-l,5-lactone yielded crude enzyme extracts in which less than 1% digilanidase activity, compared to a control, could be detected. Under the same conditions the glucosyltransferase still reached activity levels of about 90 % of that in controls without gluconic acid-l,5-lactone. © 1990, Gustav Fischer Verlag, Stuttgart. All rights reserved.