Preparation of silybin phase II metabolites: Streptomyces catalyzed glucuronidation

Flavonolignan silybin is a major component of the silymarin complex isolated from seeds of the milk thistle (Silybum marianum) having strong antioxidant and hepatoprotective effects, and also anticancer, chemoprotective, dermatoprotective and hypocholesterolemic activities. Natural silybin (silibinin in pharmacological literature) is a mixture of two diastereomers: silybin A and silybin B. Their metabolism is strongly linked to Phase II biotransformations and respective conjugates are rapidly excreted in bile and urine. Conjugation reactions of both silybins are strictly stereoselective. Therefore, optically pure compounds must be used for metabolic studies. The aim of this study was to obtain the glucuronidated metabolites of both silybin A and B. Streptomyces sp. strain M52104 was found to be a highly effective tool for the preparation of silybin A-20-O-β-glucuronide, silybin B-20-O-β-glucuronide, silybin A-7-O-β-glucuronide and silybin B-7-O-β-glucuronide and minor amounts of silybin A and B-5-O-β-glucuronide. The glucuronides, which were thoroughly characterized by MS and NMR spectroscopy, can be used as invaluable authentic standards in metabolic studies of both silybin diastereomers.