Received for publication February 19, 2009.
Revised May 6, 2009.
Accepted for publication May 7, 2009.

Quantitative analysis of UGT1A and UGT2B expression levels in human livers

Abstract

UDP-glucuronosyltransferases (UGT) catalyze glucuronidation of a variety of xenobiotics and endobiotics. UGTs are divided into two families, UGT1 and UGT2. The purpose of this study was to estimate the absolute expression levels of each UGT isoform in human liver and to evaluate the interindividual variability. Real-time RT-PCR analysis was performed to determine the copy numbers of functional nine UGT1A isoforms and seven UGT2B isoforms. We noticed that not only primers but also templates as a standard for quantification should prudently be selected. Once we established appropriate conditions, the mRNA levels of each UGT isoform in 25 individual human livers were determined. UGT1A1 (0.9-138.5), UGT1A3 (0.1-66.6), UGT1A4 (0.1-143.3), UGT1A6 (1.0-70.4), UGT1A9 (0.3-132.4), UGT2B4 (0.3-615.0), UGT2B7 (0.2-97.4), UGT2B10 (0.7-253.2), UGT2B15 (0.3-107.8), and UGT2B17 (0.5-157.1) were substantially expressed (x10⁴ copy/µg RNA) with large interindividual variability. Abundant isoforms were UGT2B4 and UGT2B10, followed by UGT1A1, UGT2B15, and UGT1A6. The sum of the UGT2B mRNA levels was higher than that of UGT1A mRNA levels. Interestingly, the mRNA levels normalized with GAPDH mRNA for almost UGT isoforms that are substantially expressed in liver showed significant correlations each other. Western blot analysis was performed using antibodies specific for UGT1A1, UGT1A4, UT1A6, or UGT2B7. Correlation between the protein and mRNA levels was observed in only UGT1A1 (r = 0.488, p < 0.01). In conclusion, this study comprehensively determined the absolute values of mRNA expression of each UGT isoform in human livers and found considerable interindividual variability.

Key words: glucuronidation, UDP glucuronyltransferases