Metabolic conversion of 24-epi-brassinolide into pentahydroxylated brassinosteroid glucosides in tomato cell cultures

Two isomeric metabolites, 25-beta-D-glucopyranosyloxy-24-epi-brassinolide and 26-beta-D-glucopyranosyloxy-24-epi-brassinolide, have been formed in cell suspension cultures of Lycopersicon esculentum from exogenously applied 24-epi-brassinolide. The two-step metabolic process involved hydroxylation of the side-chain at C-25 and C-26, respectively, followed by glucosidation of the newly formed hydroxyl groups. The ratio between both metabolites was significantly altered by in vivo treatment of the cell cultures with various cytochrome P-450-specific inhibitors, indicating the involvement of two different enzyme systems. Biosynthetically prepared 25-hydroxy-24-epibrassinolide, reapplied to cell cultures, was exclusively glucosylated at the 25-hydroxyl group, strongly suggesting regiospecificity of the corresponding glucosyltransferase.