Human tissue distribution of carbonyl reductase 1 (CBR1) is quite controversial in the literature. To understand the differences, CBR1 protein abundance in human intestine, liver, and kidney has been determined using a proteomic approach with liquid chromatography-tandem mass spectrometry. The results show that CBR1 distribution in the 3 tissues is relatively similar, within 2- to 3-folds of each other. Intestine has the highest CBR1 enzyme level (106 pmol/mg protein) followed by liver (76 pmol/mg protein) and kidney (39 pmol/mg protein). The high abundance of CBR1 in the intestine and kidney suggests the critical role of this enzyme in gut first-pass metabolism and extra-hepatic clearance. CBR1 is also detected, for the first time, in the microsomal fractions with 5- to 17-fold lower amounts than cytosols of the corresponding tissues. The average CBR1 protein amount is 14 pmol/mg protein in human hepatocytes. Individual variability of CBR1 is about 4-fold based on 13 lots of cryopreserved human hepatocytes. The relative expression factor between human liver cytosol and human recombinant CBR1 enzyme (hr-CBR1) is 0.0042. The relative activity factor between human liver cytosol and hr-CBR1 is 0.040, suggesting that significant amount of CBR1 protein in the hr-CBR1 has no catalytic activity.
Keywords: LC-MS/MS; carbonyl reductase 1; intestine; kidney; liver; protein quantification; proteomics; tissue distribution.
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