Abstract
The human UDP-glucuronosyltransferase (UGT) enzymes UGT1A9 and UGT2B7 play important roles in the hepatic glucuronidation of many drugs. The presence of bovine serum albumin (BSA) during in vitro assays was recently reported to lower the Km values of both these UGTs for their aglycone substrates without affecting the corresponding Vmax values. Nonetheless, using the specific substrates entacapone and zidovudine (AZT) for UGT1A9 and UGT2B7, respectively, and using an improved ultrafiltration method for measuring drug binding to BSA and to biological membranes, we found that the presence of BSA during the glucuronidation reaction leads to a large increase in the Vmax value of UGT1A9, in addition to lowering its Km value. On the other hand, in the case of UGT2B7, our results agree with the previously described effect of BSA, namely lowering the Km value without a large effect on the enzyme's Vmax value. The unexpected BSA effect on UGT1A9 was independent of the expression system because it was found in a recombinant enzyme that was expressed in baculovirus-infected insect cells as well as in the native enzyme in human liver microsomes. Moreover, the effect of BSA on the kinetics of 4-methylumbelliferone glucuronidation by recombinant UGT1A9 was similar to its effect on entacapone glucuronidation. Contrary to the aglycone substrates, the effect of BSA on the apparent Km of UGT1A9 for the cosubstrate UDP-α-d-glucuronic acid was nonsignificant. Our findings call for further investigations of the BSA effects on different UGTs and the inhibitors that it may remove.
Footnotes
This study was supported by the Graduate School in Pharmaceutical Research, Academy of Finland (Project Number 120975); the Sigrid Juselius Foundation; and a Helsinki University Research Foundation grant for young researchers.
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.111.041418.
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The online version of this article (available at http://dmd.aspetjournals.org) contains supplemental material.
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ABBREVIATIONS:
- UGT
- UDP-glucuronosyltransferase
- AZT
- zidovudine (3′-azido-3′-deoxythymidine)
- BSA
- bovine serum albumin
- 3D
- three-dimensional
- HEK
- human embryonic kidney
- HLM
- human liver microsomes
- HPLC
- high-performance liquid chromatography
- 4-MU
- 4-methylumbelliferone
- NSBf
- nonspecific binding to the filter device
- UDPGA
- UDP-α-d-glucuronic acid
- UPLC
- ultraperformance liquid chromatography
- MeOH
- methanol
- fu
- fraction unbound.
- Received June 28, 2011.
- Accepted August 18, 2011.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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