Simultaneous Absolute Protein Quantification of Carboxylesterases 1 and 2 in Human Liver Tissue Fractions using Liquid Chromatography-Tandem Mass Spectrometry
ABSTRACT The aims of this study were to develop a robust method for simultaneous quantification of carboxylesterases (CESs) 1 and 2 and to quantify those absolute protein levels in human liver tissue fractions. Unique peptide fragments of CES1 and CES2 in tryptically digested human liver microsomes (HLMs) and cytosol (HLC) were simultaneously quantified by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) using corresponding stable isotope-labeled peptides as internal standards. Bovine serum albumin was used as a blank matrix for the calibration curve samples. Our procedure showed good digestion efficiency, sensitivity, linearity of calibration curve, and reproducibility. The protein levels of CES1 and CES2 in 16 individual HLMs varied 4.7-fold (171-801 pmol/mg) and 3.5-fold (16.3-57.2 pmol/mg), respectively, that are approximately 10 times higher than the expression levels in HLC. The CES1/CES2 level ratio varied substantially from 3.0 to 25, and the correlation between the protein levels of CES1 and CES2 was negative, indicating significant interindividual variability and independence in their expression levels. CES1 levels significantly correlated with hydrolysis of the CES1 substrates, clopidogrel (5 μM) and oxybutynin (10 μM), whereas CES2 levels correlated strongly with hydrolysis of the CES2 substrate, irinotecan (1 μM), indicating that quantified protein levels are highly reliable. This is the first report to demonstrate the absolute protein levels of CESs quantified by LC-MS/MS.
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ABSTRACT: A new ratiometric fluorescent probe derived from 4-hydroxy-N-butyl-1,8-naphthalimide (HNN) has been developed for selective detection of human carboxylesterase 2 (hCE2). The probe is designed by introducing benzoyl moiety to HNN, based on the intramolecular charge transfer (ICT) mechanism. The probe displays satisfying stability under physiological pH conditions with very low background fluorescence signal, but it can be rapidly hydrolyzed by hCE2 and release of HNN which leads to a remarkable red shift in emission spectra (148 nm). The newly designed probe exhibits excellent selectivity towards hCE2 over other human hydrolases, while the interference from various biologically relevant chemicals can be negligible. Its potential biological applications including inhibitor screening using human tissue preparations as enzyme sources, as well as fluorescence imaging of endogenous hCE2 in human living cells, have also been demonstrated.Sensors and Actuators B Chemical 12/2014; 205:151–157. DOI:10.1016/j.snb.2014.08.066 · 3.84 Impact Factor
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ABSTRACT: Dabigatran etexilate (DABE) is an oral prodrug that is rapidly converted by esterases to dabigatran (DAB), a direct inhibitor of thrombin. To elucidate the esterase-mediated metabolic pathway of DABE, a high-performance liquid chromatography/mass spectrometry based metabolite identification and semi-quantitative estimation approach was developed. To overcome the poor full-scan sensitivity of conventional triple quadrupole mass spectrometry, precursor-product ion pairs were predicted to search for the potential in vitro metabolites. The detected metabolites were confirmed by the product ion scan. A dilution method was introduced to evaluate the matrix effects on tentatively identified metabolites without chemical standards. Quantitative information on detected metabolites was obtained using "metabolite standards" generated from incubation samples that contain a high concentration of metabolite in combination with a correction factor for mass spectrometry response. Two in vitro metabolites of DABE (M1 and M2) were identified, and quantified by the semi-quantitative estimation approach. It is noteworthy that CES1 converts DABE to M1 while CES2 mediates the conversion of DABE to M2. M1 and M2 were further metabolized to DAB by CES2 and CES1, respectively. The approach presented here provides a solution to a bioanalytical need for fast identification and semi-quantitative estimation of CES metabolites in preclinical samples.Analytical and Bioanalytical Chemistry 12/2012; 405(5). DOI:10.1007/s00216-012-6576-4 · 3.58 Impact Factor
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ABSTRACT: Targeted quantitative proteomics using heavy isotope dilution techniques is increasingly being used to quantify proteins, including UGT enzymes, in biological matrices. Here we present a multiplexed method using nanoLC-MS/MS and multiple reaction monitoring (MRM) to quantify fourteen UGT1As and UGT2Bs in liver matrices. Where feasible, we employ two or more proteotypic peptides per protein, with only four proteins quantified with only one proteotypic peptide. We apply the method to analysis of a library of 60 human liver microsome (HLM) and matching S9 samples. Ten of the UGT isoforms could be detected in liver and the expression of each was consistent with mRNA expression reported in the literature. UGT2B17 was unusual in that ~30 % of liver microsomes had no or little (<0.5 pmol/mg protein) content, consistent with a known common polymorphism. Liver S9 UGT concentrations were approximately 10 15 % those of microsomes. The method was robust, precise and reproducible, and provides novel UGT expression data in human liver that will benefit rational approaches to evaluating metabolism in drug development.Journal of Proteome Research 08/2013; 12(10). DOI:10.1021/pr4004213 · 5.00 Impact Factor