Influence of mutations associated with Gilbert and Crigler-Najjar type II syndromes on the glucuronidation kinetics of bilirubin and other UDP-glucuronosyltransferase 1A substrates

OBJECTIVES: UGT1A1 coding region mutations, including UGT1A1*6 (G71R), UGT1A1*7 (Y486D), UGT1A1*27 (P229Q) and UGT1A1*62 (F83L), have been linked to Gilbert syndrome in Asian populations, whereas homozygosity for UGT1A1*7 is associated with the Crigler-Najjar syndrome type II. This work compared the effects of (a) the individual UGT1A1 mutations on the glucuronidation kinetics bilirubin, beta-estradiol, 4-methylumbelliferone (4MU) and 1-naphthol (1NP), and (b) the Y486 mutation, which occurs in the conserved carboxyl terminal domain of UGT1A enzymes, on 4MU, 1NP and naproxen glucuronidation by UGT1A3, UGT1A6 and UGT1A10. METHODS: Mutant UGT1A cDNAs were generated by site-directed mutagenesis and the encoded proteins were expressed in HEK293 cells. The glucuronidation kinetics of each substrate with each enzyme were characterized using specific high-performance liquid chromatography (HPLC) methods. RESULTS: Compared with wild-type UGT1A1, in-vitro clearances for bilirubin, beta-estradiol, 4MU and 1NP glucuronidation by UGT1A1*6 and UGT1A1*27 were reduced by 34-74%, most commonly as a result of a reduction in Vmax. However, the magnitude of the decrease in the in-vitro clearances varied from substrate to substrate with each mutant. The glucuronidation activities of UGT1A1*7 and UGT1A1*62 were reduced by >95%. Introduction of the Y486D mutation essentially abolished UGT1A6 and UGT1A10 activities, and resulted in 60-90% reductions in UGT1A3 in-vitro clearances. CONCLUSIONS: The glucuronidation of all UGT1A1 substrates is likely to be impaired in subjects carrying the UGT1A1*6 and UGT1A1*62 alleles, although the reduction in metabolic clearance might vary with the substrate. The Y486D mutation appears to greatly reduce most, but not all, UGT1A activities.     

Attenuation of paraquat-induced motor behavior and neurochemical disturbances by L-valine in vivo

Alterations of motor behavioral patterns and monoamine contents in the discrete rat brain areas after acute paraquat exposure (3, 5, 10, 20 mg/kg, s.c.) have been studied. The results showed that paraquat at the doses of 5, 10, and 20 mg/kg significantly reduced locomotive, stereotypic, and rotational behaviors. Significant decreases of norepinephrine (NE) contents in cortex and hypothalamus, as well as striatal contents of dopamine (DA) and its acidic metabolites, were detected. In addition, L-valine (200 mg/kg, i.p.) significantly attenuated paraquat-induced toxicity at moderate dose (5 mg/kg) but not at high dose (20 mg/kg). The results provide evidence that paraquat can enter the brain as illustrated by the alterations in the motor behavioral pattern and neurochemical contents. Furthermore, the attenuation effect of L-valine against systemic administration of paraquat-induced motor behaviors was detected, with a slightly protective effect on paraquat-induced neurochemical alterations.     

No direct hepatotoxic potential following a multiple-low dose paraquat exposure in rat as related to its bioaccumulation

Paraquat (PQ) is a well-known toxic bipyridyl herbicide commonly used in agricultural countries. Pulmonary toxicity is the main cause of death but damage to other organs has also been reported. PQ is also classified as a “direct hepatotoxicant” following an acute high dose exposure. The evidence of multi-low dose toxicity of PQ was scarce. Therefore, the aim of this study was to examine the effect of multiple low doses of PQ on the liver function and xenobiotic-metabolizing enzyme activities including CYP1A1, 2E1, and 3A4, and to correlate the effects with its tissue accumulation. PQ, at the dose range 4.0–6.0 mg/kg day, was subcutaneously administered to male Wistar rats for seven consecutive days. The prominent feature of toxic response was lung toxicity. Interestingly, PQ-treatment caused a dose- and time-dependent reduction of plasma transaminase activity. Hypobilirubinemia and hypoalbuminemia were also observed without significant alteration in the liver morphology. Of all the xenobiotic-metabolizing enzymes being studied, only the activity of CYP1A1-related 7-ethoxyresorufin-O-deethylase was reduced following the highest dose of PQ administration. Plasma and tissue concentrations and accumulation of PQ analyzed by HPLC were dose-dependent showing much higher concentration (approximately 13 times) in the lung than that in the liver whereas it was undetectable in the plasma at the same time point.

It can be concluded that multi-low dose PQ might affect certain synthetic function of the liver or activity of some hepatic xenobiotic-metabolizing enzymes. Minimal PQ accumulation in the liver is one of the explanations for the lack of cytotoxic hepatic injury in this study. Plasma PQ concentration may not be a good marker of exposure and toxicity after a prolonged exposure to PQ.     

Optimizing bacterial expression of catalytically active human cytochromes P450: comparison of CYP2C8 and CYP2C9

1. Methods for the co-expression in Escherichia coli of human cytochrome P450 (CYP) 2C8 and CYP2C9 with NADPH-cytochrome P450 reductase (OxR) to produce a catalytically active system were compared. 2. Approaches assessed were expression of a CYP:OxR fusion construct, bicistronic plasmids, simultaneous transformation with CYP and OxR plasmids, and separate expression of CYP and OxR with reconstitution of activity by mixing the bacterial membranes. Two N-terminal modifications (Delta3-20 and 17alpha-leader) of the individual P450s were additionally investigated. 3. Each approach gave efficient expression of CYP2C8 and CYP2C9, but the bicistronic constructs under the expression conditions used gave low OxR expression and low catalytic activity. CYP expression was higher with the Delta3-20 construct for CYP2C9 and with the 17alpha-presequence construct for CYP2C8. 4. Using torsemide as substrate, all methods gave catalytically active systems with K(m) values similar to human liver microsomes. Mixing bacterial membranes containing separately expressed CYP and OxR reconstituted a catalytically active system with the Delta3-20 construct for CYP2C9 but not for CYP2C8, and with neither of the 17alpha- presequence constructs. OxR co-expressed with CYP in the same membrane interacted with CYP to reconstitute activity more effectively than addition of exogenous OxR membranes. 5. Expression construct and OxR co-expression strategy should be individualized for CYP isoforms.     

Cholinesterase activity,pesticide exposure and health impact in a population exposed to organophosphates

Objective  Biological monitoring is an essential component for assessing the exposure of individuals to organophosphate pesticides. The aim of this study was to determine cholinesterase activity, pesticide exposure and health effects in the exposed population. Methods  A total of 90 individuals occupationally exposed to OPs and 30 controls were recruited in this study. Erythrocyte acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activities were measured in two periods of low- and high-exposure. Results  There were statistically significant decrease in AChE and BuChE activities in the high-exposure period (20.73 ± 0.99 U/gHb and 3.73 ± 0.19 U/mL, respectively, P < 0.001) compared to the low-exposure period (29.81 ± 1.19 U/gHb and 4.92 ± 0.19 U/mL, respectively). All enzyme activities in the exposed group were statistically lower than in the control group. Analysis of the relation between cholinesterase activity and symptoms showed significant evidences. Conclusions  The results suggested the association between occupational pesticide exposure and inhibition of cholinesterases. Thus, medical monitoring of cholinesterase inhibition and intervention programs regarding safety practices during field work are important issues aimed at minimizing adverse health effects of pesticide.     

Cytotoxic and inflammatory responses of TiO2 nanoparticles on human peripheral blood mononuclear cells

Titanium dioxide nanoparticles (TiO₂‐NPs) have been widely used in many applications. Owing to their nanoscale size, interactions between cells and NPs have been expansively investigated. With the health concerns raised regarding the adverse effects of these interactions, closer examination of whether TiO₂‐NPs can induce toxicity towards human cells is greatly needed. Therefore, in this study, we investigated the cytotoxicity of TiO₂‐NPs towards human blood cells (peripheral blood mononuclear cells [PBMCs]) in serum‐free medium, for which there is little information regarding the cytotoxic effects of TiO₂‐NPs. Our results provide evidence that PBMCs treated with TiO₂‐NPs (at concentrations ≥25 μg ml^?1) for 24 h significantly reduced cell viability and significantly increased production of toxic mediators such as reactive oxygen species and inflammatory response cytokines such as interleukin‐6 and tumor necrosis factor‐α (P < 0.05). Cell apoptosis induction also occurred at these concentrations. Significant expressions of cyclooxygenase‐2 and interleukin‐1β were also observed in PBMCs treated with TiO₂‐NPs at concentrations ≥125 μg ml^?1. Our data presented here clearly indicate that the concentration of TiO₂‐NPs (at size ~26.4 ± 1.2 nm) applied to human blood cells has a strong impact on cytotoxic induction. Copyright © 2016 John Wiley & Sons, Ltd.     

The influence of metabolic gene polymorphisms on urinary 1-hydroxypyrene concentration in Thai bus drivers

Polycyclic aromatic hydrocarbons (PAHs) are associated with an increased cancer risk. CYP1A1 and GSTs enzymes are important in metabolism of PAHs. Genetic polymorphisms of these enzymes are responsible for enzyme activity and concentration variation. The objectives of this study were to evaluate association of 1-OHP concentration with genetic polymorphisms of CYP1A1 and GSTs in Thai bus drivers. The results showed that 1-OHP levels in bus drivers were significantly higher than that in the control group. Significant difference in 1-OHP was found between smokers and non-smokers, in only bus drivers. Significantly increasing of 1-OHP levels were observed in bus drivers with CYP1A1 MspI and exon 7 variants. Whereas, bus drivers with GSTP1 Val and GSTM1 null genotypes showed decreasing in excretion of 1-OHP. No association between 1-OHP and polymorphisms of GSTT1 was found. This study indicated that 1-OHP concentrations were associated with exposure to air pollution, cigarette smoking and polymorphisms of CYP1A1, GSTM1 and GSTP1 genes.     

In vivo evaluation of coumarin and nicotine as probe drugs to predict the metabolic capacity of CYP2A6 due to genetic polymorphism in Thais

The association between the distribution characteristics of CYP2A6 catalytic activities toward nicotine and coumarin, and the frequency distribution of CYP2A6 variant alleles reported was estimated in 120 healthy Thais. The distributions of the subjects as classified by the amounts of 7-hydroxycoumarin (7-OHC) excreted in the urine and by cotinine/nicotine ratio in the plasma were clearly bimodal. However, the numbers of apparently poor metabolizers for coumarin and nicotine were different. The inter-individual variability in the in vivo dispositions of coumarin and nicotine closely related to the CYP2A6 genetic polymorphism. There was a close correlation between the rate of 7-OHC excretion in the urine and cotinine/nicotine ratio in the plasma among subjects (R=0.92, p<0.001). The frequency of CYP2A6 allele found in the present study was: CYP2A6*1A=32% (95% CI, 22.1-39.4%), CYP2A6*1B=27% (95% CI, 19.4-33.5%), CYP2A6*9=20% (95% CI, 17.6-23.3%), CYP2A6*4=14% (95% CI, 9.6-17.8%), CYP2A6*7=5% (95% CI, 3.7-9.4%), CYP2A6*10=2% (95% CI, 0.8-5.1%). Subjects having CYP2A6*1A/*1B were found to have a higher rate of 7-OHC excretion, as well as a higher cotinine/nicotine ratio in the plasma compared with those of the other genotypes. In contrast, subjects with CYP2A6*4/*7 and CYP2A6*7/*7 almost lacked any cotinine formation, whereas urinary 7-OHC was still detectable. CYP2A6*9 allele clearly resulted in reduced enzyme activities. Despite the absence of the homozygote for CYP2A6*10 allele, the presence of CYP2A6*10 allele significantly decreased the enzyme activities. The results of the present study demonstrate that in vivo phenotyping of CYP2A6 using nicotine and coumarin are not metabolically equivalent. Nicotine is a better probe according to its specificity, while coumarin is still valuable to be used for a routine CYP2A6 phenotyping since the test employs a non-invasive method.     

Impaired absorption of paracetamol in vegetarians.

1. The absorption and disposition of paracetamol was investigated in 10 healthy male Thai vegetarians and 10 similar non-vegetarians following an oral dose of 20 mg kg-1. 2. The absorption rate of paracetamol was significantly impaired in the vegetarians compared with the non-vegetarians as shown by a lower mean Cmax (11.7 +/- 1.4 vs 15.6 +/- 1.6 mg l-1; 95% confidence interval of the difference 2.49 to 5.36), increased tmax (median 1.75, range 0.75 to 3 h compared with 0.75 and 0.25 to 2 h) and an increase in the time for input of 50% of the total amount absorbed (0.54 +/- 0.38 compared with 0.20 +/- 0.10 h; 95% confidence interval of the difference 0.063 to 0.61). 3. A significantly lower total 24 h urinary recovery of paracetamol and metabolites (72.1 +/- 5.4 vs 86.4 +/- 5.4% of the dose; 95% confidence interval of the difference 8.0 to 20.6) indicated a decrease in the extent of absorption in the vegetarians also, although the total AUC values did not differ significantly between the two groups. 4. The plasma paracetamol half-life, partial metabolic clearances and fractional urinary excretion of the glucuronide, sulphate, cysteine and mercapturic acid conjugates of paracetamol were similar in the vegetarians and non-vegetarians.