Glycosylation in cardenolide biosynthesis

Abstract

The glycosylation and deglycosylation of cardiac glycosides was investigated using cell suspension cultures and shoot cultures, both established from Digitalis lanata EHRH. plants, as well as isolated enzymes. Shoots were capable of glucosylating digitoxigenin, evatromonoside, digiproside, glucodigitoxigenin and digitoxin. Suspension cultured Digitalis cells glucosylated all the substrates mentioned but digiproside, whereas the UDP-glucosedependent cardenolide glucosyltransferase isolated from that source did not accept digitoxigenin and digiproside as substrates. It is concluded that at least three different glucosyltransferases are involved in cardiac glycoside formation in Digitalis. Similar experiments carried out with glucosylated cardenolides which were administered to cultured cells, shoots and a cardenolide/~-glucosidase isolated from young leaves revealed that at least two different glucosidases occur in Digitalis lanata, albeit in different tissues or during different phases of development. The biotransformation of glucoevatromonoside was investigated using unlabelled compound and [lac-glucose]glucoevatromonoside synthesized enzymatically. After 7 d of incubation almost no radioactivity could be recovered from the cardenolide fraction, indicating that the terminal glucose of glucoevatromonoside was now incorporated into volatile, hydrophilic and insoluble compounds. Since, on the other hand, large amounts of cardenolides were found in the experiments with unlabelled glucoevatromonoside it is assumed that steady state or pool size regulation is achieved by the coordinated action of a cardenolide glucosidase and a glucosyltransferase.Abbreviations: Acdox -D-acetyldigitoxose, dgen-digoxigenin, dox-D-digitoxose, dten -digitoxigenin, dtl -Ddigitalose, fuc-D-fucose, gten-gitoxigenin, qun-D-quinovose, CGH-cardenolide 16'-O-glucohydrolase, DFT -UDP-fucose:digitoxigenin 3-O-fucosyltransferase, DGT-UDP-glucose:Digitoxin 16'-O-glucosyltransferase, DQT -UDP-quinovose:digitoxigenin 3-O-quinovosyltransferase

The biosynthesis of steroids and triterpenoids

“…Formation of the butenolide ring (117) has been suggested to involve transfer of a malonyl moiety to the 21-hydroxy group to form a malonyl hemi-ester (116) (catalysed by malonyl-coenzyme A: 21-hydroxypregnane 21-hydroxy-malonyltransferase) and subsequent Claisen-type condensation of the malonyl moiety with the C-20 ketone, accompanied by decarboxylation 1224 (rather than Claisen-type condensation as the initial step, as previously assumed). Enzymes involved in the final glycosylation stage have been described from Digitalis species 1225,1226 and Nerium oleander. 1227 The biosynthesis of 5 -cardenolides in Asclepias curassavica has also been studied [1228][1229][1230] and the topic of cardenolide biosynthesis has been reviewed.…”

Calotropin: Natural Phytomolecules for Cutting-edge Features

“…[5,21,29] Most cardiac glycosides embrace one to four sugar residues connected to the genin, increasing their capability to bind to the heart muscle and make them water-soluble. [30,31] Numerous distinct substitution patterns were discovered at the sugar moiety in the calotropis family [Figure 3]. The hydroxyl group can be altered by being acetylated or epimerized at C-3′ and subjected to its oxidation.…”

The effect of gamma ray and sugars on increasing the concentration of cardiac glycoside compounds of Digitalis lanata In vitro

“…The reason for this may be due to its being a double sugar that disintegrates during the process of sterilizing the food medium containing it into two units of Glucose sugar, and as it is known that glucose sugar enters into the formation of primary cardiac glycosides such as Lanatoside C, Lanatoside B, and Lanatoside A, which in turn are converted during the drying process of plant parts to Digoxin, Gitoxin, Digitoxin respectively [16]. For this reason, it was observed that the presence of Maltose sugar in the food medium had the greatest effect on the formation of cardiac glycoside compounds.…”