Characterization of feline cytochrome P450 2B6

Altered hepatic cytochrome P450 expression in cats after chronic exposure to decabromodiphenyl ether (BDE-209)

The knowledge of cytochrome P450 (CYP) expression involved in chemical exposure are necessary in clinical applications for the medication and prediction of adverse effects. The aim of this study was to evaluate the mRNA expression of CYP1–CYP3 families in cats exposed to BDE-209 for one year. All selected CYP isoforms showed no significant difference in mRNA expressions between control and exposure groups, however, CYP3A12 and CYP3A131 revealed tend to be two times higher in the exposure group compared to control group. The present results indicate that the chronic exposure of BDE209 could not alter CYP expression in the liver of cats. This result considered caused by the deficiency of CYP2B subfamily which is major metabolism enzyme of polybrominated diphenyl ethers (PBDEs) in cat.

Tissue distribution and characterization of feline cytochrome P450 genes related to polychlorinated biphenyl exposure

Cats have been known to be extremely sensitive to chemical exposures. To understand these model species' sensitivity to chemicals and their toxicities, the expression profiles of xenobiotic-metabolizing enzymes should be studied. Unfortunately, the characterization of cytochrome P450 (CYP), the dominant enzyme in phase I metabolism, in cats has not extensively been studied. Polychlorinated biphenyls (PCBs) are known as CYP inducers in animals, but the information regarding the PCB-induced CYP expression in cats is limited. Therefore, in the present study, we aimed to elucidate the mRNA expression of the CYP1-CYP3 families in the cat tissues and to investigate the CYP mRNA expression related to PCB exposure. In cats, the greatest abundance of CYP1-CYP3 (CYP1A2, CYP2A13, CYP2C41, CYP2D6, CYP2E1, CYP2E2, CYP2F2, CYP2F5, CYP2J2, CYP2U1, and CYP3A132) was expressed in the liver, but some extrahepatic isozymes were found in the kidney (CYP1A1), heart (CYP1B1), lung (CYP2B11 and CYP2S1) and small intestine (CYP3A131). In cats, CYP1A1, CYP1A2 and CYP1B1 were significantly upregulated in the liver as well as in several tissues exposed to PCBs, indicating that these CYPs were distinctly induced by PCBs. The strong correlations between 3,3',4,4'-tetrachlorobiphenyl (CB77) and CYP1A1 and CYP1B1 mRNA expressions were noted, demonstrating that CB77 could be a potent CYP1 inducer. In addition, these CYP isoforms could play an essential role in the PCBs biotransformation, particularly 3-4 Cl-PCBs, because a high hydroxylated metabolite level of 3-4 Cl-OH-PCBs was observed in the liver.

Xenobiotica-metabolizing enzymes in the lung of experimental animals, man and in human lung models

The xenobiotic metabolism in the lung, an organ of first entry of xenobiotics into the organism, is crucial for inhaled compounds entering this organ intentionally (e.g. drugs) and unintentionally (e.g. work place and environmental compounds). Additionally, local metabolism by enzymes preferentially or exclusively occurring in the lung is important for favorable or toxic effects of xenobiotics entering the organism also by routes other than by inhalation. The data collected in this review show that generally activities of cytochromes P450 are low in the lung of all investigated species and in vitro models. Other oxidoreductases may turn out to be more important, but are largely not investigated. Phase II enzymes are generally much higher with the exception of UGT glucuronosyltransferases which are generally very low. Insofar as data are available the xenobiotic metabolism in the lung of monkeys comes closed to that in the human lung; however, very few data are available for this comparison. Second best rate the mouse and rat lung, followed by the rabbit. Of the human in vitro model primary cells in culture, such as alveolar macrophages and alveolar type II cells as well as the A549 cell line appear quite acceptable. However, (1) this generalization represents a temporary oversimplification born from the lack of more comparable data; (2) the relative suitability of individual species/models is different for different enzymes; (3) when more data become available, the conclusions derived from these comparisons quite possibly may change.

Kinetics of Cyclophosphamide Metabolism in Humans, Dogs, Cats, and Mice and Relationship to Cytotoxic Activity and Pharmacokinetics

Cyclophosphamide (CP), a prodrug that is enzymatically converted to the cytotoxic 4-hydroxycyclophosphamide (4OHCP) by hepatic enzymes, is commonly used in both human and veterinary medicine to treat cancers and modulate the immune system. We investigated the metabolism of CP in humans, dogs, cats, and mice using liver microsomes; apparent K M, V max, and intrinsic clearance (V max/K M) parameters were estimated. The interspecies and intraspecies variations in kinetics were vast. Dog microsomes were, on average, 55-fold more efficient than human microsomes, 2.8-fold more efficient than cat microsomes, and 1.2-fold more efficient than mouse microsomes at catalyzing CP bioactivation. These differences translated to cell-based systems. Breast cancer cells exposed to 4OHCP via CP bioactivation by microsomes resulted in a stratification of cytotoxicity that was dependent on the species of microsomes measured by IC50: dog (31.65 μM), mouse (44.95 μM), cat (272.6 μM), and human (1857 μM). The contributions of cytochrome P450s, specifically, CYP2B, CYP2C, and CYP3A, to CP bioactivation were examined: CYP3A inhibition resulted in no change in 4OHCP formation; CYP2B inhibition slightly reduced 4OHCP in humans, cats, and mice; and CYP2C inhibition drastically reduced 4OHCP formation in each species. Semiphysiologic modeling of CP metabolism using scaled metabolic parameters resulted in simulated data that closely matched published pharmacokinetic profiles, determined by noncompartmental analysis. The results highlight differential CP metabolism delineated by species and demonstrate the importance of metabolism on CP clearance.

Identification and quantification of domestic feline cytochrome P450 transcriptome across multiple tissues

Understanding of cytochrome P450 (CYP) isoform distribution and function in the domestic feline is limited. Only a few studies have defined individual CYP isoforms across metabolically relevant tissues, hampering the ability to predict drug metabolism and potential drug-drug interactions. Using RNA sequencing (RNA-seq), transcriptomes from the 99 Lives Cat Genome Sequencing Initiative databank combined with experimentally acquired whole transcriptome sequencing of healthy, adult male (n = 2) and female (n = 2) domestic felines, expression of 42 CYP isoforms were identified in 20 different tissues. Thirty-seven of these isoforms had not been previously reported in cats. Depending on the tissue, three to twenty-nine CYP isoform transcripts were expressed. The feline genome annotations did not differentiate CYP2E1 and 2E2 genes, demonstrating poor annotation for this gene using the reference genome. As the majority of the sequences are based on automated pipelines, complete cDNA sequences for translation into CYP protein sequences could not be determined. This study is the first to identify and characterize 37 additional CYP isoforms in feline tissues, increasing the number of identified CYP from the previously reported seven isoforms to 42 across 20 tissues.

Limited expression of functional cytochrome p450 2c subtypes in the liver and small intestine of domestic cats

1. Compared to information for herbivores and omnivores, knowledge on xenobiotic metabolism in carnivores is limited. The cytochrome P450 2C (CYP2C) subfamily is recognized as one of the most important CYP groups in human and dog. We identified and characterized CYP2C isoforms and variants in cat, which is an obligate carnivore. 2. Quantitative RT-PCR and immunoblot analyses were carried out to evaluate the expression of CYP2C in the liver and small intestine. A functional CYP2C isoform was heterologously expressed in yeast microsomes to determine the enzymatic activity. 3. Cat had two CYP2C genes, 21 and 41, in the genome; however, CYP2C21P was a pseudogene that had many stop codons. Three splicing variants of CYP2C41 were identified (v1-v3), but only one of them (v1) showed a complete deduced amino acid sequence as CYP2C protein. Transcripts of feline CYP2C41v1 were detected but the amounts were negligible or very small in the liver and small intestine. Immunoreactivity to an antihuman CYP2C antibody was confirmed in the recombinant feline CYP2C41v1 but not in the feline liver. 4. Recombinant feline CYP2C41v1 metabolized several substrates, including dibenzylfluorescein that is specific to human CYP2C. 5. The results suggest a limited role of functional CYP2C isoforms in xenobiotic metabolism in cat.

Cytochrome P450 2B diversity in a dietary specialist—the red tree vole (Arborimus longicaudus)

Although herbivores rely on liver enzymes to biotransform plant secondary metabolites ingested in plant-based diets, only a few enzymes from a handful of species have been characterized at the genomic level. In this study, we examined cytochrome P450 2B (CYP2B) sequence diversity and gene copy number in a conifer specialist, the red tree vole (Arborimus longicaudus). We fed captive individuals exclusively Douglas-fir (Pseudotsuga menziesii) foliage, cloned and sequenced their liver CYP2B cDNA, and estimated CYP2B gene copy number. We identified 21 unique CYP2B nucleotide sequences, and 20 unique CYP2B amino acid sequences. Gene copy number of CYP2B was estimated at 7.7 copies per haploid genome. We compared red tree vole CYP2B with CYP2B sequences of a generalist, the prairie vole (Microtus ochrogaster), found in GenBank. Our study revealed that the CYP2B enzymes of red tree voles possess unique sequences compared to CYP2B enzymes of other herbivorous species. The unique combination of amino acid residues at key substrate recognition sites of CYP2B enzymes may underlie the ability of the red tree vole to specialize on a highly toxic diet of Douglas-fir.

Comparison of predicted intrinsic hepatic clearance of 30 pharmaceuticals in canine and feline liver microsomes

1. Known cytochrome P450 (CYP) substrates in humans are used in veterinary medicine, with limited knowledge of the similarity or variation in CYP metabolism. Comparison of canine and feline CYP metabolism via liver microsomes report that human CYP probes and inhibitors demonstrate differing rates of intrinsic clearance (CL_int). 2. The purpose of this study was to utilize a high-throughput liver microsome substrate depletion assay, combined with microsomal and plasma protein binding to compare the predicted hepatic clearance (CL_hep) of thirty therapeutic agents used off-label in canines and felines, using both the well-stirred and parallel tube models. 3. In canine liver microsomes, 3/30 substrates did not have quantifiable CL_int, while midazolam and amitriptyline CL_int was too rapid for accurate determination. A CL_hep was calculated for 29/30 substrates in feline microsomes. Overall, canine CL_hep was faster compared to the feline, with fold differences ranging from 2-20-fold. 4. A comparison between the well-stirred and parallel tube model indicates that the parallel tube model reports a slighter higher CL_hep in both species. 5. The differences in CYP metabolism between canine and feline highlight the need for additional research into CYP expression and specificity.