Cloning and functional analysis of 3β-hydroxysteroid dehydrogenase gene involved in biosynthesis of digitoxin

Abstract

Digitoxin, an important secondary metabolite of Digitalis purpurea, is a commonly used cardiotonic in clinical practice. 3β-Hydroxysteroid dehydrogenase(3βHSD) is a key enzyme involved in the biosynthesis of digitoxin. It belongs to the short-chain dehydrogenase/reductase(SDR) family, playing a role in the biosynthesis of cardiac glycosides by oxidizing and isomerizing the precursor sterol. In this study, two 3βHSD genes were cloned from D. purpurea. The results showed that the open reading frame(ORF) of Dp3βHSD1 was 780 bp, encoding 259 amino acid residues. The ORF of Dp3βHSD2 was 774 bp and encoded 257 residues. Dp3βHSD1/2 had the cofactor binding site TGxxxA/GxG and the catalytic site YxxxK. In vitro experiments confirmed that Dp3βHSD1/2 catalyzed the generation of progesterone from pregnenolone, and Dp3βHSD1 had stronger catalytic capacity than Dp3βHSD2. The expression level of Dp3βHSD1 was much higher than that of Dp3βHSD2 in leaves, and digitoxin was only accumulated in leaves. The results implied that Dp3βHSD1 played a role in the dehydrogenation of pregnenolone to produce progesterone in the biosynthesis of digitoxin. This study provides a reference for further exploring the biosynthetic pathway of cardiac glycosides in D. purpurea.