Abstract
Chemoenzymatic glucuronidation of the optically pure silybin A (1) using ovine liver glucuronyl transferase afforded three β-glucuronides of silybin, substituted at phenolic OH groups at the positions C-20 (2), C-7 (3), and C-5 (4) formed in the yields 27, 62.5, and 2.5%, respectively. Using these standards, it was shown that the main silybin conjugate in humans is its 20-β-d-glucuronate (2), while the C-7 regioisomer (3) was formed in lower proportion. The rate of conjugation of (natural) silybin diastereomers 10S, 11S and 10R, 11R, and therefore also their metabolism in humans is rather different. The radical scavenging activity of 2 is considerably lower than that of its aglycone (1); however, the activity of 3 is higher than in the silybin. These findings corroborate the hypothesis that, at physiological pH, the exclusive target for one-electron oxidation of the silybin molecule is theo-methoxy-phenolic structure at C-19, C-20. This is first pharmacological study using optically pure silybin.
Footnotes
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Send reprint requests to: Vladimı́r Kr̆en, Institute of Microbiology, Academy of Sciences of the Czech Republic, Laboratory of Biotransformation, Vı́deňská 1083, CZ 142 20 Prague 4, Czech Republic. E-mail: kren{at}biomed.cas.cz
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This work was supported by Grants 303/98/0414 and 303/99/1382 from the Grant Agency of the Czech Republic; Grant PP-Z1/13/96 from Ministry of Industry and Commerce of the Czech Republic; Grant VS 96021, MSM 151100003 from Ministry of Education, Youth and Sport of the Czech Republic; and by New Zealand Foundation for Research Science and Technology Contract C08620.
- Abbreviations used are::
- UDPGT
- uridine 5′-diphosphoglucuronyl transferase
- DPPH
- 1,1-diphenyl-2-picrylhydrazyl radical
- UDPGA
- uridine 5′-diphosphoglucuronic acid trisodium salt
- gCOSY
- gradient-selected correlated spectroscopy
- NOESY
- nuclear Overhauser and exchange spectroscopy
- ROESY
- rotation frame nuclear Overhauser enhancement spectroscopy
- HMQC
- heteronuclear multiple quantum correlation
- HMBC
- heteronuclear multiple bond correlation
- HOM2DJ
- homonuclear J-resolved two-dimensional NMR
- Received May 22, 2000.
- Accepted September 14, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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