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Herein, we evaluated CYPs and their nuclear receptor mRNA induction by exposure to typical inducers, omeprazole, rifampicin, and phenobarbital in cynomolgus monkey hepatocytes. Six freshly-isolated hepatocytes and 6 cryopreserved hepatocytes from cynomolgus monkey liver were prepared for a 14-day monolayer culture, 28-day co-culture with feeder cells, and 28-day 3D spheroid culture with feeder cells. Omeprazole and rifampicin respectively induced CYP1A1 and CYP3A8 mRNAs, while phenobarbital induced CYP2C43, CYP2C75, and CYP3A8, and slightly induced CYP2B6. The nuclear receptors AHR, PXR, and CAR mRNA levels, which were activated by omeprazole, rifampicin, and phenobarbital, respectively, tended to decrease via exposure to inducers despite the increase in CYP mRNA levels. These trends were similar for all three culture methods. No evident difference was observed in CYP mRNA induction between fresh and cryopreserved hepatocytes. Based on mRNA levels, the co-culture and 3D spheroid culture methods are more reasonable than monolayer culture for CYP evaluation, because the use of feeder cells can reduce the number of hepatocytes, improve the cell adhesion, and maintain the mRNA expression levels. In addition, co-culture method is more cost-effective, as common culture plates can be used.

Background: We previously showed that hormetic responses can be established in cell activity tests using human and murine adherent cells. This time, we examined whether hormetic responses can be established in cell proliferation tests using suspended human and murine lymphoid cells. Methods: Human lymphoblastoid cells (TK6) and mouse lymphoma cells (L5178Y) were cultured in multi-well culture plates and treated with mitomycin C, ethyl methansulfonate, hygromycin B, aclarubicin or colchicine at various dose levels and the number of cells was measured at varied times using a flow cytometer. Results: When the ratio of the number of cells treated with a test chemical to those in the negative control was plotted, the dose-response relationship typically showed a reverse U-shaped curve, indicating the occurrence of hormesis and existence of thresholds in cell toxicity. The hormetic responses depended largely on the test chemical, dose level and exposure time. When examining responses over the course of time, a J-shaped or fallen S-shaped curve was also observed. Conclusions: The dose-response relationship showed a reverse U-shaped curve, a hallmark of hormesis, at least some time points for all chemicals tested here, indicating that chemical hormesis can be established in in vitro cell proliferation tests.

In this study, freshly isolated and cryopreserved cynomolgus monkey hepatocytes were seeded on Cell-able® plates with feeder cells to form spheroids and were cultured for 28 days. As a control, hepatocytes were also cultured with or without feeder cells on collagen-coated plates. We verified the mRNA expression levels of drug-metabolizing enzyme-related genes and the leakage of enzymes (AST, ALT, LDH, and γ-GTP) as indicators of cell survival. As a result, the patterns of target mRNA expression in fresh and cryopreserved hepatocytes were very similar during the culture period between culture methods. mRNA expression levels were highly maintained at day 28 using the 3D spheroid and co-culture methods, demonstrating that these methods are useful for maintenance of liver function. Leakage of AST and ALT was higher at day 3 but decreased at day 14. LDH was not detected, suggesting that the cell viability was also maintained during the culture period. Furthermore, the functional differences between fresh and cryopreserved hepatocytes were not clearly detected. The co-culture method was useful for long-term culture not requiring 3D structure, and the 3D spheroid culture method was effective as well. With these techniques, cynomolgus monkey hepatocytes are expected to exhibit smaller individual differences and high reproducibility.

In preclinical studies, the cynomolgus monkey (CM) model is frequently used to predict the pharmacokinetics of drugs in the human small intestine, because of its evolutionary closeness to humans. Intestinal organoids that mimic the intestinal tissue have attracted attention in regenerative medicine and drug development. In this study, we generated intestinal organoids from CM induced pluripotent stem (CMiPS) cells and analyzed their pharmacokinetic functions. CMiPS cells were induced into the hindgut; then, the cells were seeded on microfabricated culture vessel plates to form spheroids. The resulting floating spheroids were differentiated into intestinal organoids in a medium containing small-molecule compounds. The mRNA expression of intestinal markers and pharmacokinetic-related genes was markedly increased in the presence of small-molecule compounds. The organoids possessed a polarized epithelium and contained various cells constituting small intestinal tissues. The intestinal organoids formed functional tight junctions and expressed drug transporter proteins. In addition, in the organoids generated, cytochrome P450 3A8 (CYP3A8) activity was inhibited by the specific inhibitor ketoconazole and was induced by rifampicin. Therefore, in the present work, we successfully generated intestinal organoids, with pharmacokinetic functions, from CMiPS cells using small-molecule compounds.

The common marmoset is a promising species for evaluating the safety of drug candidates. To further understand the capacity for drug metabolism in marmosets, a cDNA encoding a CYP2B enzyme was cloned from the total RNA fraction of marmoset liver by 3'- and 5'-RACE methods. Nucleotide and deduced amino acid sequences showed 90.8 and 86.2% identity, respectively, with human CYP2B6. The marmoset CYP2B6 (marCYP2B6) protein was expressed in insect cells, and its enzymatic properties were compared with those of human (humCYP2B6) and cynomolgus monkey (cynCYP2B6) orthologs in liver and insect cell microsomes. Enzymatic functions were examined for the oxidation of 7-ethoxy-4-(trifluoromethyl)coumarin (7-ETC), bupropion (BUP) and efavirenz (EFV). The kinetic profiles for the oxidation of the three substrates by liver microsomal fractions were similar between humans and cynomolgus monkeys (biphasic for 7-ETC and monophasic for BUP and EFV), but that of marmosets was unique (monophasic for 7-ETC and biphasic for BUP and EFV). Recombinant enzymes, humCYP2B6 and cynCYP2B6, also yielded similar kinetic profiles for the oxidation of the three substrates, whereas marCYP2B6 showed activity only for 7-ETC hydroxylation. In silico docking simulations suggested that two amino acid residues, Val-114 and Leu-367, affect the activity of marCYP2B6. In fact, a marCYP2B6 mutant with substitutions V114I and L367V exhibited BUP hydroxylase activity that was 4-fold higher than that of humCYP2B6, while its EFV 8-hydroxylase activity was only 10% that of the human enzyme. These results indicate that the amino acids at positions 114 and 367 affect the enzymatic capacity of marmoset CYP2B6.

The general aim of the present study is to discriminate between mouse genotoxic and non-genotoxic hepatocarcinogens via selected gene expression patterns in the liver as analyzed by quantitative real-time PCR (qPCR) and statistical analysis. qPCR was conducted on liver samples from groups of 5 male, 9-week-old B6C3F(1) mice, at 4 and 48h following a single intraperitoneal administration of chemicals. We quantified 35 genes selected from our previous DNA microarray studies using 12 different chemicals: 8 genotoxic hepatocarcinogens (2-acetylaminofluorene, 2,4-diaminotoluene, diisopropanolnitrosamine, 4-dimethylaminoazobenzene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, N-nitrosomorpholine, quinoline and urethane) and 4 non-genotoxic hepatocarcinogens (1,4-dichlorobenzene, dichlorodiphenyltrichloroethane, di(2-ethylhexyl)phthalate and furan). A considerable number of genes exhibited significant changes in their gene expression ratios (experimental group/control group) analyzed statistically by the Dunnett's test and Welch's t-test. Finally, we distinguished between the genotoxic and non-genotoxic hepatocarcinogens by statistical analysis using principal component analysis (PCA) of the gene expression profiles for 7 genes (Btg2, Ccnf, Ccng1, Lpr1, Mbd1, Phlda3 and Tubb2c) at 4h and for 12 genes (Aen, Bax, Btg2, Ccnf, Ccng1, Cdkn1a, Gdf15, Lrp1, Mbd1, Phlda3, Plk2 and Tubb2c) at 48h. Seven major biological processes were extracted from the gene ontology analysis: apoptosis, the cell cycle, cell proliferation, DNA damage, DNA repair, oncogenes and tumor suppression. The major, biologically relevant gene pathway suggested was the DNA damage response pathway, resulting from signal transduction by a p53-class mediator leading to the induction of apoptosis. Eight genes (Aen, Bax, Btg2, Ccng1, Cdkn1a, Gdf15, Phlda3 and Plk2) that are directly associated with Trp53 contributed to the PCA. The current findings demonstrate a successful discrimination between genotoxic and non-genotoxic hepatocarcinogens, using qPCR and PCA, on 12 genes associated with a Trp53-mediated signaling pathway for DNA damage response at 4 and 48 h after a single administration of chemicals.

Aims UDP-glucuronosyltransferase 1A1 (UGT1A1) plays important roles in the glucuronidation of various drugs and endogenous substances. Cynomolgus monkeys are regarded as experimental animals closer to humans in studies on safety evaluation and biotransformation for drug development. In this study, the similarities and differences in the enzymatic properties of UGT1A1 between humans and cynomolgus monkeys were precisely identified. Main methods Human and cynomolgus monkey UGT1A1s (humUGT1A1 and monUGT1A1, respectively) were cloned, and the corresponding proteins were heterologously expressed in insect cells. The enzymatic properties of UGT1A1 proteins were characterized by kinetic analysis of 7-hydroxy-4-trifluoromethylcoumarin (7-HFC), estradiol at 3-hydroxy position (E-3OH) and 7-ethyl-10-hydroxycamptothecin (SN-38) glucuronidation. Key findings There were no significant differences in the levels of kinetic parameters for 7-HFC, E-3OH and SN-38 glucuronidation between humans and cynomolgus monkeys in both enzyme sources of liver microsomes and recombinant UGT1A1s. 7-HFC and E-3OH glucuronidation by human liver microsomes exhibited biphasic and sigmoidal kinetics, respectively, whereas the kinetics by cynomolgus monkey liver microsomes fitted the typical Michaelis–Menten model. SN-38 glucuronidation by human and cynomolgus monkey liver microsomes exhibited autoactivation kinetics. In recombinant UGT1A1 enzymes expressed in insect cells, the kinetics of 7-HFC, E-3OH and SN-38 glucuronidation fitted the substrate inhibition (7-HFC glucuronidation) or Hill equation (E-3OH and SN-38 glucuronidation), and each glucuronidation showed the same kinetic profile between humans and cynomolgus monkeys. Significance These findings suggest that the enzymatic properties of human and cynomolgus monkey UGT1A1 enzymes are very similar.

We investigated the change of the mRNA levels of sulfotransferase and UDP-glucuronosyltransferase isoforms by the prototypical microsomal enzyme inducers rifampicin (Rif), dexamethasone (Dex), and omeprazole (Ome) in primary cultures of cryopreserved human and cynomolgus monkey hepatocytes. Real-time RT-PCR analysis was performed using primers and TaqMan probes. Rif, Dex, and Ome increased SULT2A1 mRNA level in both human and cynomolgus monkey hepatocytes in dose-dependent manner, but not SULT1A1 mRNA level. Rif, Dex, and Ome increased the mRNA level of UGT1A1 in both human and cynomolgus monkey hepatocytes, Ome more potently in humans and Rif and Ome more potently in monkeys. They also increased the mRNA levels of UGT1A6 and UGT1A9 in cynomolgus monkey hepatocytes, though the extent of elevation of UGT1A6 and UGT1A9 mRNA levels was smaller than that of UGT1A1 mRNA level. Furthermore, these inducers scarcely affected UGT1A6 and UGT1A9 in human hepatocytes. Rif, Dex, and Ome also showed no remarkable effect on the mRNA levels of UGT2Bs in human or cynomolgus monkey hepatocytes. We also studied in detail the time course of mRNA expression of these enzymes in primary cultures of hepatocytes. In conclusion, the results of the present study show that primary cultures of hepatocytes isolated from the cynomolgus monkey liver are as useful as human hepatocytes for evaluating the induction of drug-metabolizing enzymes in preclinical studies.

Quantitative real-time RT-PCR was used to investigate the effects of prototypical drug-metabolizing enzyme inducers rifampicin (Rif), dexamethasone (Dex), and omeprazole (Ome) on mRNA expression levels of the housekeeping genes β-actin (ACTB), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), β-glucuronidase (GUSB), hypoxanthine phosphoribosyltransferase 1 (HPRT1), peptidylprolylisomerase A (PPIA), TATA box binding protein (TBP), and transferrin receptor (TFRC) in primary cultures of cryopreserved human and rat hepatocytes. The mRNA levels of ACTB, GAPDH, GUSB, PPIA, TBP, and TFRC relative to HPRT1 in human hepatocytes were constant at all concentrations of inducers. However, the mRNA level of GAPDH relative to HPRT1 in rat hepatocytes was markedly increased by Rif. The mRNA levels of GAPDH, GUSB, PPIA, TBP, and TFRC relative to HPRT1 in rat hepatocytes were significantly increased by Dex. ACTB and HPRT1 are suitable internal controls for evaluating mRNA expression levels in primary cultures of human and rat hepatocytes after Rif, Dex, or Ome exposure.

As part of a collaborative study by the Mammalian Mutagenicity Study Group of the Environmental Mutagen Society of Japan, we examined micronucleus induction in hepatocytes following oral administration of 2,6-dinitrotoluene (2,6-DNT) at 30, 40, and 50 mg/kg/day for 14 days or at 20, 30, and 40 mg/kg/day for 28 days to young adult male rats. This compound is known to be a rat liver carcinogen. The formation of micronucleated hepatocytes was confirmed to be dose-dependent with statistically significant increases observed in both treatments. In contrast, no statistically significant changes in the percentage of micronucleated immature erythrocytes were observed in any dose group in the bone marrow micronucleus assay. These results indicated that the repeated-dose liver micronucleus assay has the potential to detect genotoxic hepatocarcinogens and can be integrated into general toxicological studies.

We investigated the utility of three-dimensional (3D) spheroid cultures of human hepatocytes in discovering drug metabolites. Metabolites of acetaminophen, diclofenac, lamotrigine, midazolam, propranolol and salbutamol were analyzed by liquid chromatography-tandem mass spectrometry (LC/MS/MS) to measure enzyme activities in this system cultured for 2 and 7 days. Sequential metabolic reactions by Phase I and then Phase II enzymes were found in diclofenac [CYP2C9 and UDP-glucuronyltransferases (UGTs)], midazolam (CYP3A4 and UGTs) and propranolol (CYP1A2/2D6 and UGTs). Moreover, lamotrigine and salbutamol were metabolized to lamotrigine-N-glucuronide and salbutamol 4-O-sulfate, respectively. These metabolites, which are human specific, could be observed in clinical studies, but not in conventional hepatic culture systems as in previous reports. Acetaminophen was metabolized to glucuronide and sulfate conjugates, and N-acetyl-p-benzo-quinoneimine (NAPQI) and its metabolites were not observed. In addition, mRNA of drug-metabolism enzymes [CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, UGT1A1, UGT2B7, sulfotransferase 1A1 (SULT1A1) and glutathione S-transferase pi 1 (GSTP1)], which were measured by qRT-PCR, were expressed in the human hepatocyte spheroids. In conclusion, these results suggest that human hepatocyte spheroids are useful in discovering drug metabolites.

The potential genotoxicity of the rodent liver carcinogen 2,6-dinitrotoluene (2,6-DNT) was evaluated in compliance with the guidelines for genotoxicity studies of drugs (Notification No. 1604, Nov. 1, 1999, Ministry of Health and Welfare, Japan) and the OECD guidelines for the testing of chemicals by performing the bacterial reverse mutation (Ames) assay, the in vitro chromosomal aberration assay, and the in vivo comet assay (alkaline single cell gel electrophoresis) in rat liver. In the Ames assay, 2,6-DNT was moderately positive irrespective of metabolic activation. In the in vitro chromosomal aberration assay, under conditions where the test substance would precipitate out, weak structural aberrations were observed with or without S9 mix at each dose at which the cell growth rate was about 40 to 50%. The in vivo comet assay yielded positive results in rat liver; that is to say, the increases in % tail DNA in liver in the 25 and 50 mg/kg groups were observed statistically significantly and dose-dependent. Our findings are in accordance with previous findings in the in vivo/in vitro unscheduled DNA synthesis (UDS) assay in rat liver and in a young rat liver micronucleus assay, although the rat bone marrow micronucleus assay gave negative results. These results suggest that test systems using liver are a useful method for the in vivo genotoxicity assessment of chemicals that require metabolic activation.

Toxic and pharmacokinetic profiles of drug candidates are evaluated in vivo often using monkeys as experimental animals, and the data obtained are extrapolated to humans. Well understanding physiological properties, including drug-metabolizing enzymes, of monkeys should increase the accuracy of the extrapolation. The present study was performed to compare regio- and stereoselectivity in the oxidation of propranolol (PL), a chiral substrate, by cytochrome P450 2D (CYP2D) enzymes among humans, cynomolgus monkeys and marmosets. Complimentary DNAs encoding human CYP2D6, cynomolgus monkey CYP2D17 and marmoset CYP2D19 were cloned, and their proteins expressed in a yeast cell expression system. The regio- and stereoselective oxidation of PL enantiomers by yeast cell microsomal fractions were compared. In terms of efficiency of expression in the system, the holo-proteins ranked CYP2D6=CYP2D17>>CYP2D19. This may be caused by the bulky side chain of the amino acid residue at position 119 (leucine for CYP2D19 vs. valine for CYP2D6 and CYP2D17), which can disturb the incorporation of the heme moiety into the active-site cavity. PL enantiomers were oxidized by all of the enzymes mainly into 4-hydroxyproranolol (4-OH-PL), followed by 5-OH-PL and N-desisopropylpropranolol (NDP). In the kinetic analysis, apparent K(m) values were commonly in the μM range and substrate enantioselectivity of R-PL

Pairs of forward and reverse primers and TaqMan probes specific to each of 19 drug-metabolizing enzymes (cytochrome P450s, UDP-glucuronosyltransferases, glutathione S-transferases, and sulfotransferases) and 5 transporters (ABC and SLC transporters) in the cynomolgus monkey were prepared. The expression level of each target mRNA was analyzed in total RNA obtained from three specimens of various cynomolgus monkey tissues (adrenal gland, brain, heart, kidney, large intestine, liver, lung, pancreas, prostate, small intestine, spleen, testis, and thymus) by real-time reverse transcription PCR using an Applied Biosystems 7500 Fast Real-Time PCR System. The data obtained in the present study provide useful information on tissue-specific profiles of the expression of these target mRNAs in the cynomolgus monkey, and the results are expected to be valuable in establishing drug metabolism- and transporter-mediated screening systems using the cynomolgus monkey for the evaluation of new chemical entities in new drug development.

This study investigated the changes in the mRNA levels of the ATP binding cassette (ABC) transporters multidrug resistance 1 (MDR1), multidrug resistance-associated protein 1 (MRP1), and multidrug resistance-associated protein 2 (MRP2) following exposure to the prototypical microsomal enzyme inducers rifampicin (Rif), dexamethasone (Dex), and omeprazole (Ome) in primary cultures of cryopreserved human and cynomolgus monkey hepatocytes. Analysis was performed by real-time reverse transcription-polymerase chain reaction using primers and TaqMan probes. First, the time course of the mRNA expression of these transporters in primary cultures of human and cynomolgus monkey hepatocytes was examined in detail. The ratio of MDR1 and MRP2 mRNA to beta-actin mRNA in both human and cynomolgus monkey hepatocytes remained constant from 48 to 72 h and from 24 to 72 h of culture, respectively. Second, the hepatocytes were exposed to the inducers and the changes in the levels of the transporter mRNAs were examined. Rif increased MDR1 and MRP1 mRNA levels in both human and cynomolgus monkey hepatocytes, while Ome slightly increased MDR1 and MRP1 mRNA levels in cynomolgus monkey hepatocytes. Rif and Ome increased MRP2 mRNA levels in both human and cynomolgus monkey hepatocytes. In contrast, Dex tended to decrease the mRNA levels of MDR1, MRP1, and MRP2 in both human and cynomolgus monkey hepatocytes. Cynomolgus monkey hepatocytes appeared to be more responsive than human hepatocytes to the inducers. These results indicate that primary cultures of cynomolgus monkey hepatocytes are as useful as primary cultures of human hepatocytes for evaluating the induction of MDR1, MRP1, and MRP2 mRNAs in preclinical studies.

The stereoselective glucuronidation of propranolol (PL) in human and cynomolgus monkey liver microsomes, and the roles of human hepatic UDP-glucuronosyltransferase (UGT) isofoms involved in the enantiomeric glucuronidation of PL using recombinant UGT enzymes were investigated. In Michaelis-Menten plots, R- and S-PL glucuronidation by human liver microsomes showed sigmoidal kinetics whereas the kinetics of enantiomeric PL glucuronidation by cynomolgus monkey liver microsomes was monophasic. The Km, Vmax and CLint values of cynomolgus monkey liver microsomes were generally higher than the S50, Vmax and CLmax values of human liver microsomes in R- and S-PL glucuronidation. The glucuronidation of R- and S-PL was catalyzed by at least 3 UGT isoforms: UGT1A9, UGT2B4 and UGT2B7. Michaelis-Menten plots for R- and S-PL glucuronidation by UGT1A9 were monophasic, whereas the kinetics of UGT2B7 showed sigmoidal curves. Enantiomeric R-PL glucuronidation by UGT2B4 showed sigmoidal kinetics, whereas S-PL glucuronidation displayed monophasic kinetics. UGT1A9 showed remarkable stereoselectivity in Vmax and CLint values of R-PL < S-PL. These findings demonstrate that the profiles of enantiomeric PL glucuronidation in human and cynomolgus monkey liver microsomes are largely different and suggest that the human hepatic UGT isofoms UGT1A9, UGT2B4 and UGT2B7 play distinctive roles in enantiomeric PL glucuronidation.

This study was conducted to investigate the effects of treatment with the prototypical inducers rifampicin (Rif), dexamethasone (Dex), and omeprazole (Ome) on the mRNA levels of drug-metabolizing enzymes in primary cultures of cryopreserved human, cynomolgus monkey, and rat hepatocytes. Analysis was performed by quantitative real-time RT-PCR using primers and TaqMan probes. Treatment with Ome substantially increased the mRNA levels of both CYP1A1 and CYP1A2 in human hepatocytes, but increased only the mRNA level of CYP1A1 in monkey hepatocytes, whereas it had no marked effect on the mRNA levels of CYP1A1 or CYP1A2 in rat hepatocytes. Treatment with Rif or Dex did not markedly affect the mRNA level of CYP1A in any of the hepatocyte cultures under the conditions used. All three inducers increased the mRNA level of CYP3A8 in monkey hepatocytes (in the order Rif>Dex>or=Ome), and a similar profile was observed for the mRNA level of CYP3A4 in human hepatocytes, but the potency of induction was markedly attenuated. In contrast, only Dex substantially increased the mRNA level of CYP3A1 in rat hepatocytes, with Rif and Ome showing no effects. These results indicate that the molecular mechanisms responsible for the regulation of CYP1A2 genes differ between humans and cynomolgus monkeys, although the regulatory mechanisms for CYP1A1 and CYP3A genes are similar.

The mRNA induction of various transporters by rifampicin (Rif), dexamethasone (Dex) and omeprazole (Ome) was investigated in primary cultures of cryopreserved human and rat hepatocytes. Analysis was performed by quantitative real-time RT-PCR using primers and TaqMan probes. In primary cultures of human hepatocytes, mRNA levels of MDR and MRP1 were increased by about 1.5 fold and 1.3 fold, respectively, by exposure to Rif at 2 to 50 microM as compared with 0.1% DMSO-treated controls. MRP2 mRNA levels in the same human hepatocytes were significantly increased by 1.2 to 1.8 fold by exposure to Rif at 50 microM as compared with controls. In primary cultures of rat hepatocytes, Mdr1a and Mdr1b mRNA levels were not increased or only slightly increased at 24 hr by exposure to any of the inducers at 2, 10 or 50 microM. Mrp2 mRNA levels in the same rat hepatocytes were significantly increased by 7 to 45 fold by exposure to Dex at 2 microM as compared with controls. Based on the species differences observed in the present study, primary cultures of cryopreserved hepatocytes from both the human and rat should be useful in preclinical drug development for evaluating candidate drugs for transporter induction.

UDP-glucuronosyltransferase 1A6 (UGT1A6) is a major isoform in the human liver that glucuronidates numerous drugs, environmental chemicals and endogenous substrates. In this study, human and cynomolgus monkey UGT1A6 cDNAs (humUGT1A6 and monUGT1A6, respectively) were cloned, and the corresponding proteins were heterologously expressed in yeast cells to identify the functions of primate UGT1A6s. The enzymatic properties of UGT1A6 proteins were characterized by the kinetic analysis of serotonin (5-hydroxytryptamine, 5-HT) and 4-methylumbelliferone (4-MU) glucuronidation. humUGT1A6 and monUGT1A6 showed 96% identity in their nucleotide and amino acid sequences. Immunoblotting analysis using an antibody raised against human UGT1A6 showed that protein staining intensities were different between human and cynomolgus monkey UGT1A6 enzymes in microsomal fractions from livers and yeast cells, although both enzymes were detectable. The apparent K(m) value (15 mM) for 5-HT glucuronidation of cynomolgus monkey liver microsomes was significantly higher than that (8.6mM) of human liver microsomes, whereas V(max) values were lower in cynomolgus monkeys (2.8 nmol/min/mg protein) than in humans (8.6 nmol/min/mg protein). No significant species difference was observed in K(m) (approximately 90 microM) or V(max) (approximately 25 nmol/min/mg protein) values for liver microsomal 4-MU glucuronidation. In yeast cell microsomes, K(m) values (approximately 6mM) for 5-HT glucuronidation by recombinant UGT1A6s were similar, while a V(max) value (0.1nmol/min/mg protein) of monUGT1A6 was significantly lower than that (0.7 nmol/min/mg protein) of humUGT1A6. In 4-MU glucuronidation, both K(m) (210 microM) and V(max) (3.5 nmol/min/mg protein) values of monUGT1A6 were significantly higher than those of humUGT1A6 (K(m), 110 microM; V(max), 1.5nmol/min/mg protein). These findings suggest that the enzymatic properties of UGT1A6 were extensively different between humans and cynomolgus monkeys, although humUGT1A6 and monUGT1A6 showed high homology at the amino acid level. The information gained in this study should help with in vivo extrapolation and to assess the toxicity of xenobiotics.