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Preparation of a Specific Monoclonal Antibody against Human UDP-Glucuronosyltransferase (UGT) 1A9 and Evaluation of UGT1A9 Protein Levels in Human Tissues

Glucuronidation is a major detoxification pathway of drugs and xenobiotics that are catalyzed by the UDP-glucuronosyltransferase (UGT) superfamily. Determination of the protein levels of the individual UGT isoforms in human tissues is required for the successful extrapolation of in vitro metabolic data to in vivo clearance. Most previous studies evaluating UGT isoform expression were limited to the mRNA level because of the high degree of amino acid sequence homology between UGT isoforms that has hampered the availability of isoform-specific antibodies. In this study, we generated a peptide-specific monoclonal antibody against human UGT1A9. We demonstrated that this antibody does not cross-react with the other UGT1A isoforms including UGT1A7, UGT1A8, and UGT1A10 and shows a high degree of amino acid sequence similarity with UGT1A9. Using this antibody, we found that UGT1A9 protein is expressed in the kidney and the liver but not in the jejunum or the ileum, consistent with previous reports of mRNA expression. In a panel of 20 individual human livers, the UGT1A9 protein levels exhibited 9-fold variability. It is noteworthy that the relative UGT1A9 protein levels were not correlated with the UGT1A9 mRNA level (r = −0.13), like other UGT isoforms reported previously, suggesting the importance of evaluating UGT isoform expression at protein levels. In conclusion, we generated a specific monoclonal antibody against UGT1A9 and evaluated the distribution and relative expression levels of the UGT1A9 protein in human tissues. This antibody may serve as a useful tool for further studies of UGT1A9 to evaluate its physiological, pharmacological, and toxicological roles in human tissues.