Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Cryo-EM Structure of Recombinantly Expressed hUGDH Unveils a Hidden, Alternative Allosteric Inhibitor

Human UDP-glucose dehydrogenase (hUGDH) catalyzes the oxidation of UDP-glucose into UDP-glucuronic acid, an essential substrate in the Phase II metabolism of drugs. hUGDH is a hexamer that exists in an equilibrium between an active (E) state and an inactive (EΩ) state, with the latter being stabilized by the binding of the allosteric inhibitor UDP-xylose (UDP-Xyl). The allosteric transition between EΩ and E is slow and can be observed as a lag in progress curves. Previous analysis of the lag suggested that unliganded hUGDH exists mainly as EΩ, but two unique crystal forms suggest that the enzyme favors the E state. Resolving this discrepancy is necessary to fully understand the allosteric mechanism of hUGDH. Here, we used cryo-EM to show that recombinant hUGDH expressed in Escherichia coli copurifies with UDP-4-keto-xylose (UX4O), which mimics the UDP-Xyl inhibitor and favors the EΩ state. Cryo-EM studies show that removing UX4O from hUGDH shifts the ensemble to favor the E state. This shift is consistent with progress curve analysis, which shows the absence of a lag for unliganded hUGDH. Inhibition studies show that hUGDH has similar affinities for UDP-Xyl and UX4O. The discovery that UX4O inhibits allosteric hUGDH suggests that UX4O may be the physiologically relevant inhibitor of allosteric UGDHs in bacteria that do not make UDP-Xyl.

Glucuronidation of tizoxanide, an active metabolite of nitazoxanide, in liver and small intestine: Species differences in humans, monkeys, dogs, rats, and mice and responsible UDP-glucuronosyltransferase isoforms in humans

Tizoxanide (TZX) is an active metabolite of nitazoxanide (NTZ) originally developed as an antiparasitic agent, and is predominantly metabolized into TZX glucuronide. In the present study, TZX glucuronidation by the liver and intestinal microsomes of humans, monkeys, dogs, rats, and mice, and recombinant human UDP-glucuronosyltransferase (UGT) were examined. The kinetics of TZX glucuronidation by the liver and intestinal microsomes followed the Michaelis-Menten or biphasic model, with species-specific variations in the intrinsic clearance (CLint). Rats and mice exhibited the highest CLint values for liver microsomes, while mice and rats were the highest for intestinal microsomes. Among human UGTs, UGT1A1 and UGT1A8 demonstrated significant glucuronidation activity. Estradiol and emodin inhibited TZX glucuronidation activities in the human liver and intestinal microsomes in a dose-dependent manner, with emodin showing stronger inhibition in the intestinal microsomes. These results suggest that the roles of UGT enzymes in TZX glucuronidation in the liver and small intestine differ extensively across species and that UGT1A1 and/or UGT1A8 mainly contribute to the metabolism and elimination of TZX in humans. This study presents the relevant and novel-appreciative report on TZX metabolism catalyzed by UGT enzymes, which may aid in the assessment of the antiparasitic, antibacterial, and antiviral activities of NTZ for the treatment of various infections.

Physiologically based Kinetic Modeling-Facilitated Quantitative In Vitro to In Vivo Extrapolation to Predict the Effects of Aloe-Emodin in Rats and Humans

Aloe-emodin, a natural hydroxyanthraquinone, exerts both adverse and protective effects. This study aimed at investigating these potential effects of aloe-emodin in humans upon the use of food supplements and herbal medicines using a physiologically based kinetic (PBK) modeling-facilitated quantitative in vitro to in vivo extrapolation (QIVIVE) approach. For this, PBK models in rats and humans were established for aloe-emodin including its active metabolite rhein and used to convert in vitro data on hepatotoxicity, nephrotoxicity, reactive oxidative species (ROS) generation, and Nrf2 induction to corresponding in vivo dose-response curves, from which points of departure (PODs) were derived by BMD analysis. The derived PODs were subsequently compared to the estimated daily intakes (EDIs) resulting from the use of food supplements or herbal medicines. It is concluded that the dose levels of aloe-emodin from food supplements or herbal medicines are unlikely to induce toxicity, ROS generation, or Nrf2 activation in liver and kidney.

Screening of 16 major drug glucuronides for time-dependent inhibition of nine drug-metabolizing CYP enzymes - detailed studies on CYP3A inhibitors

Time-dependent inhibition of cytochrome P450 (CYP) enzymes has been observed for a few glucuronide metabolites of clinically used drugs. Here, we investigated the inhibitory potential of 16 glucuronide metabolites towards nine major CYP enzymes in vitro. Automated substrate cocktail methods were used to screen time-dependent inhibition of CYP1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2J2 and 3A in human liver microsomes. Seven glucuronides (carvedilol β-D-glucuronide, diclofenac acyl-β-D-glucuronide, 4-hydroxyduloxetine β-D-glucuronide, ezetimibe phenoxy-β-D-glucuronide, raloxifene 4'-glucuronide, repaglinide acyl-β-D-glucuronide and valproic acid β-D-glucuronide) caused NADPH- and time-dependent inhibition of at least one of the CYPs investigated, including CYP2A6, CYP2C19 and CYP3A. In more detailed experiments, we focused on the glucuronides of carvedilol and diclofenac, which inhibited CYP3A. Carvedilol β-D-glucuronide showed weak time-dependent inhibition of CYP3A, but the parent drug carvedilol was found to be a more potent inhibitor of CYP3A, with the half-maximal inhibitor concentration (IC50) decreasing from 7.0 to 1.1 µM after a 30-min preincubation with NADPH. The maximal inactivation constant (kinact) and the inhibitor concentration causing half of kinact (KI) for CYP3A inactivation by carvedilol were 0.051 1/min and 1.8 µM, respectively. Diclofenac acyl-β-D-glucuronide caused time-dependent inactivation of CYP3A at high concentrations, with a 4-fold IC50 shift (from 400 to 98 µM after a 30-min preincubation with NADPH) and KI and kinact values of >2,000 µM and >0.16 1/min. In static predictions, carvedilol caused significant (>1.25-fold) increase in the exposure of the CYP3A substrates midazolam and simvastatin. In conclusion, we identified several glucuronide metabolites with CYP inhibitory properties. Based on detailed experiments, the inactivation of CYP3A by carvedilol may cause clinically significant drug-drug interactions.

Hepatic glucuronidation of tetrabromobisphenol A and tetrachlorobisphenol A: interspecies differences in humans and laboratory animals and responsible UDP-glucuronosyltransferase isoforms in humans

Tetrabromobisphenol A (TBBPA) and tetrachlorobisphenol A (TCBPA), bisphenol A (BPA) analogs, are endocrine-disrupting chemicals predominantly metabolized into glucuronides by UDP-glucuronosyltransferase (UGT) enzymes in humans and rats. In the present study, TBBPA and TCBPA glucuronidation by the liver microsomes of humans and laboratory animals (monkeys, dogs, minipigs, rats, mice, and hamsters) and recombinant human hepatic UGTs (10 isoforms) were examined. TBBPA glucuronidation by the liver microsomes followed the Michaelis-Menten model kinetics in humans, rats, and hamsters and the biphasic model in monkeys, dogs, minipigs, and mice. The CLint values based on the Eadie-Hofstee plots were mice (147) > monkeys (122) > minipigs (108) > humans (100) and rats (98) > dogs (81) > hamsters (47). TCBPA glucuronidation kinetics by the liver microsomes followed the biphasic model in all species except for minipigs, which followed the Michaelis-Menten model. The CLint values were monkeys (172) > rats (151) > mice (134) > minipigs (104), dogs (102), and humans (100) > hamsters (88). Among recombinant human UGTs examined, UGT1A1 and UGT1A9 showed higher TBBPA and TCBPA glucuronidation abilities. The kinetics of TBBPA and TCBPA glucuronidation followed the substrate inhibition model in UGT1A1 and the Michaelis-Menten model in UGT1A9. The CLint values were UGT1A1 (100) > UGT1A9 (42) for TBBPA glucuronidation and UGT1A1 (100) > UGT1A9 (53) for TCBPA glucuronidation, and the activities at high substrate concentration ranges were higher in UGT1A9 than in UGT1A1 for both TBBPA and TCBPA. These results suggest that the glucuronidation abilities toward TBBPA and TCBPA in the liver differ extensively across species, and that UGT1A1 and UGT1A9 expressed in the liver mainly contribute to the metabolism and detoxification of TBBPA and TCBPA in humans.

Enhanced in vivo absorption and biodistribution of curcumin loaded into emulsions with high medium-chain triglyceride content

The health benefits of curcumin have been demonstrated by several clinical studies, but its low bioavailability compromises its functionality. In this regard, emulsions have proven to be effective encapsulation systems for curcumin. Nevertheless, emulsions with a high oil content (50%) may offer some advantages due to the large amount of compound they can incorporate. Therefore, the aim of this work was to study the pharmacokinetics and biodistribution of curcumin when carried in optimized emulsions containing 50% MCT oil and a plant-based emulsifier (soybean lecithin) at 2 h or 4 h post-oral administration to rats. The most stable emulsion was obtained using 50% of oil and a surfactant-oil-ratio 0.1, through a microfluidization process. After the oral administration of the systems (150 mg curcumin/kg body weight), curcumin glucuronide was the main compound present in plasma (AUC0-t = 1556.3 ng·h·ml-1), especially at 2-4 h post-administration. The total curcuminoid bioavailability was increased by 10.6-fold when rats were fed with the curcumin emulsion rather than with a control suspension. Moreover, rats fed with the emulsion showed the highest accumulation of free curcuminoids, which present the highest biological activity, in the liver (129 ng curcumin/g tissue) and brown adipose tissue (193 ng curcumin/g tissue). The obtained results are of great interest since the presence of curcumin in the brown adipose tissue has been shown to play a relevant role in the prevention of obesity and its related metabolic disorders.

Xanthine Dehydrogenase Is a Modulator of Dopaminergic Neurodegeneration in Response to Bacterial Metabolite Exposure in C. elegans

Oxidative stress is a contributing factor to Parkinson's disease (PD). Considering the prevalence of sporadic PD, environmental exposures are postulated to increase reactive oxygen species and either incite or exacerbate neurodegeneration. We previously determined that exposure to the common soil bacterium, Streptomyces venezuelae (S. ven), enhanced oxidative stress and mitochondrial dysfunction in Caenorhabditis elegans, leading to dopaminergic (DA) neurodegeneration. Here, S. ven metabolite exposure in C. elegans was followed by RNA-Seq analysis. Half of the differentially identified genes (DEGs) were associated with the transcription factor DAF-16 (FOXO), which is a key node in regulating stress response. Our DEGs were enriched for Phase I (CYP) and Phase II (UGT) detoxification genes and non-CYP Phase I enzymes associated with oxidative metabolism, including the downregulated xanthine dehydrogenase gene, xdh-1. The XDH-1 enzyme exhibits reversible interconversion to xanthine oxidase (XO) in response to calcium. S. ven metabolite exposure enhanced XO activity in C. elegans. The chelation of calcium diminishes the conversion of XDH-1 to XO and results in neuroprotection from S. ven exposure, whereas CaCl2 supplementation enhanced neurodegeneration. These results suggest a defense mechanism that delimits the pool of XDH-1 available for interconversion to XO, and associated ROS production, in response to metabolite exposure.

Urinary excretion of silibinin diastereoisomers and their conjugated metabolites in rat and human at different dosages

Silibinin is a mixture of two flavonoid lignan silibinins A and B from the seeds of milk thistle (Silybum marianum L.). Using UPLC/Q-TOFMS, a total of 18 metabolites were identified in rat and human urine samples after oral administration of Silibinin Capsule. Furthermore, nine glucuronides and/or sulfated metabolites and two prototype compounds were simultaneously quantified in rat urine after oral administration of Silibinin Capsule at 50 and 100 mg/kg. Over a 72-h period, 27.6% and 23.3% of the silibinin were excreted in the forms of eleven metabolites in urine, among which, 5 major metabolites, including silibinin A-7-O-β-glucuronide (SA-7G), silibinin B-7-O-β-glucuronide (SB-7G), silibinin A-5-O-β-glucuronide (SA-5G), silibinin B-5-O-β-glucuronide (SB-5G) and silibinin A-20-O-glucuronide (SA-20G), accounted for 20.5% and 15.5% of the dosages separatively at doses of 50 and 100 mg/kg. These results suggested that glucuronidation at the C7-, C5- and C20- hydroxyls was the primary metabolic pathway of silibinin diastereoisomers in vivo. The present results provide helpful information for in vivo metabolism and clinical usage of Silibinin Capsule.

Glucuronides: From biological waste to bio-nanomedical applications

Long considered as no more than biological waste meant to be eliminated in urine, glucuronides have recently contributed to tremendous developments in the biomedical field, particularly against cancer. While glucuronide prodrugs monotherapy and antibody-directed enzyme prodrug therapy have been around for some time, new facets have emerged that combine the unique properties of glucuronides notably in the fields of antibody-drug conjugates and nanomedicine. In both cases, glucuronides are utilized as a vector to improve pharmacokinetics and confer localized activation of potent drugs at tumor sites while also decreasing systemic toxicity. Here we will discuss some of the most promising strategies using glucuronides to promote successful anti-tumor therapeutic treatments.

Adverse Effects of Fusarium Toxins in Ruminants: A Review of In Vivo and In Vitro Studies

With an increased knowledge of the mechanism of action of Fusarium mycotoxins, the concept that these substances are deleterious only for monogastric species is obsolete. Indeed, most mycotoxins can be converted into less toxic compounds by the rumen microflora from healthy animals. However, mycotoxin absorption and its conversion to more toxic metabolites, as well as their impact on the immune response and subsequently animal welfare, reproductive function, and milk quality during chronic exposure should not be neglected. Among the Fusarium mycotoxins, the most studied are deoxynivalenol (DON), zearalenone (ZEN), and fumonisins from the B class (FBs). It is remarkable that there is a paucity of in vivo research, with a low number of studies on nutrient digestibility and rumen function. Most of the in vitro studies are related to the reproductive function or are restricted to rumen incubation. When evaluating the production performance, milk yield is used as an evaluated parameter, but its quality for cheese production is often overlooked. In the present review, we summarize the most recent findings regarding the adverse effects of these mycotoxins with special attention to dairy cattle.

Relationship of circulating total bilirubin, UDP-glucuronosyltransferases 1A1 and the development of non-alcoholic fatty liver disease: a cross-sectional study

Background

This study aimed to investigate the correlation of circulating total bilirubin (TB) and UGT1A1 with NAFLD in Chinese Han population.

Methods

172 adults were enrolled from the Qingdao Municipal Hospital from May 2019 to October 2020. All individuals were examined with MRI-PDFF and divided into no steatosis, mild steatosis, moderate steatosis, and severe steatosis groups according to the MRI-PDFF values. The biochemical indexes and UGT1A1 were measured.

Results

There was no significant difference of circulating TB and UGT1A1 levels between NAFLD group and controls. In the moderate steatosis and severe steatosis groups, the circulating TB levels were higher than that in control group (all P < 0.05). In addition, circulating TB levels were weak positively associated with liver fat fraction in NAFLD patients (ρ = 0.205, P = 0.001). There was no significant correlation between circulating UGT1A1 levels with liver fat fraction in patients with NAFLD (ρ = 0.080, P = 0.179), but positively correlation was found in patients with severe steatosis (ρ = 0.305, P = 0.026).

Conclusions

The circulating TB levels were significant high in patients with moderate and severe steatosis. Circulating TB levels were weakly associated with liver fat fraction in patients with NAFLD, and the circulating UGT1A1 levels were positively correlated with liver fat fraction in NAFLD patients with severe steatosis.

Trial registration: ChiCTR, ChiCTR1900022744. Registered 24 April 2019 – Retrospectively registered, http://www.chictr.org.cn/edit.aspx?pid=38304&htm=4.

Drug-oxidizing and conjugating non-cytochrome P450 (non-P450) enzymes in cynomolgus monkeys and common marmosets as preclinical models for humans

Many drug oxidations and conjugations are mediated by a variety of cytochromes P450 (P450) and non-P450 enzymes in humans and non-human primates. These non-P450 enzymes include aldehyde oxidases (AOX), carboxylesterases (CES), flavin-containing monooxygenases (FMO), glutathione S-transferases (GST), arylamine N-acetyltransferases (NAT),sulfotransferases (SULT), and uridine 5'-diphospho-glucuronosyltransferases (UGT) and their substrates include both endobiotics and xenobiotics. Cynomolgus macaques (Macaca fascicularis, an Old-World monkey) are widely used in preclinical studies because of their genetic and physiological similarities to humans. However, many reports have indicated the usefulness of common marmosets (Callithrix jacchus, a New World monkey) as an alternative non-human primate model. Although knowledge of the drug-metabolizing properties of non-P450 enzymes in non-human primates is relatively limited, new research has started to provide an insight into the molecular characteristics of these enzymes in cynomolgus macaques and common marmosets. This mini-review provides collective information on the isoforms of non-P450 enzymes AOX, CES, FMO, GST, NAT, SULT, and UGT and their enzymatic profiles in cynomolgus macaques and common marmosets. In general, these non-P450 cynomolgus macaque and marmoset enzymes have high sequence identities and similar substrate recognitions to their human counterparts. However, these enzymes also exhibit some limited differences in function between species, just as P450 enzymes do, possibly due to small structural differences in amino acid residues. The findings summarized here provide a foundation for understanding the molecular mechanisms of polymorphic non-P450 enzymes and should contribute to the successful application of non-human primates as model animals for humans.

Adenine-related compounds modulate UDP-glucuronosyltransferase (UGT) activity in mouse liver microsomes

Adenine-related compounds are allosteric inhibitors of UDP-glucuronosyltransferase (UGT) in rat liver microsomes (RLM) and human UGT isoforms treated with detergent or pore-forming peptide, alamethicin.To clarify whether the same is true beyond species, the effects of adenine-related compounds on 4-methylumbelliferone (4-MU) glucuronidation were examined using detergent-treated mouse liver microsomes (MLM).Brij-58 treatment of MLM increased the V_max and the Michaelis constant, K_m, of 4-MU. This study was performed using Brij-58-treated MLM as an enzyme source. ATP- and ADP-inhibited 4-MU glucuronidation. In contrast, AMP caused a 1.5-fold increase in glucuronidation. Oxidised forms, NAD⁺ and NADP⁺, potently inhibited 4-MU glucuronidation, whereas the reduced forms, NADH and NADPH, did not. Furthermore, the IC₅₀ values of ATP, ADP, NAD⁺, and NADP⁺ were approximately 15 μM.In our previous study, ATP was the strongest inhibitor of UGT activity in RLM. However, in this study, the above-mentioned compounds inhibited 4-MU UGT in a comparable and non-competitive manner. Furthermore, AMP antagonised the inhibitory effects of ATP and ADP.These results suggest that ATP, ADP, NAD⁺, and NADP⁺ are common endogenous inhibitors of UGT beyond species.

Metabolism and biomarkers of heterocyclic aromatic amines in humans

Heterocyclic aromatic amines (HAAs) form during the high-temperature cooking of meats, poultry, and fish. Some HAAs also arise during the combustion of tobacco. HAAs are multisite carcinogens in rodents, inducing cancer of the liver, gastrointestinal tract, pancreas, mammary, and prostate glands. HAAs undergo metabolic activation by N-hydroxylation of the exocyclic amine groups to produce the proposed reactive intermediate, the heteroaryl nitrenium ion, which is the critical metabolite implicated in DNA damage and genotoxicity. Humans efficiently convert HAAs to these reactive intermediates, resulting in HAA protein and DNA adduct formation. Some epidemiologic studies have reported an association between frequent consumption of well-done cooked meats and elevated cancer risk of the colorectum, pancreas, and prostate. However, other studies have reported no associations between cooked meat and these cancer sites. A significant limitation in epidemiology studies assessing the role of HAAs and cooked meat in cancer risk is their reliance on food frequency questionnaires (FFQ) to gauge HAA exposure. FFQs are problematic because of limitations in self-reported dietary history accuracy, and estimating HAA intake formed in cooked meats at the parts-per-billion level is challenging. There is a critical need to establish long-lived biomarkers of HAAs for implementation in molecular epidemiology studies designed to assess the role of HAAs in health risk. This review article highlights the mechanisms of HAA formation, mutagenesis and carcinogenesis, the metabolism of several prominent HAAs, and the impact of critical xenobiotic-metabolizing enzymes on biological effects. The analytical approaches that have successfully biomonitored HAAs and their biomarkers for molecular epidemiology studies are presented.

Inhibition of human UDP-glucuronosyltransferase enzyme by Dabrafenib: Implications for drug-drug interactions

Dabrafenib is a novel small molecule tyrosine kinase inhibitor (TKI) which is used to treat metastatic melanoma. The aim of this research was to survey the effects of dabrafenib on human UDP-glucuronosyltransferases (UGTs) and to evaluate the risk of drug-drug interactions (DDIs). The formation rates for 4-methylumbelliferone (4-MU) glucuronide and trifluoperazine-glucuronide in 12 recombinant human UGT isoforms with or without dabrafenib were measured and HPLC was used to investigate the inhibitory effects of dabrafenib on UGTs. Inhibition kinetic studies were also conducted. In vitro-in vivo extrapolation approaches were further used to predict the risk of DDI potentials of dabrafenib via inhibition of UGTs. Our data indicated that dabrafenib had a broad inhibitory effect on 4-MU glucuronidation by inhibiting the activities of UGTs, especially on UGT1A1, UGT1A7, UGT1A8, and UGT1A9, and dabrafenib could increase the area under the curve of co-administered drugs. Dabrafenib is a strong inhibitor of several UGTs and the co-administration of dabrafenib with drugs primarily metabolized by UGT1A1, 1A7, 1A8 or 1A9 may induce potential DDIs.

Clinical Phenotypes of Parkinson's Disease Associate with Distinct Gut Microbiota and Metabolome Enterotypes

Parkinson’s disease (PD) is a clinically heterogenic disorder characterized by distinct clinical entities. Most studies on motor deficits dichotomize PD into tremor dominant (TD) or non-tremor dominant (non-TD) with akinetic-rigid features (AR). Different pathophysiological mechanisms may affect the onset of motor manifestations. Recent studies have suggested that gut microbes may be involved in PD pathogenesis. The aim of this study was to investigate the gut microbiota and metabolome composition in PD patients in relation to TD and non-TD phenotypes. In order to address this issue, gut microbiota and the metabolome structure of PD patients were determined from faecal samples using 16S next generation sequencing and gas chromatography–mass spectrometry approaches. The results showed a reduction in the relative abundance of Lachnospiraceae, Blautia, Coprococcus, Lachnospira, and an increase in Enterobacteriaceae, Escherichia and Serratia linked to non-TD subtypes. Moreover, the levels of important molecules (i.e., nicotinic acid, cadaverine, glucuronic acid) were altered in relation to the severity of phenotype. We hypothesize that the microbiota/metabolome enterotypes associated to non-TD subtypes may favor the development of gut inflammatory environment and gastrointestinal dysfunctions and therefore a more severe α-synucleinopathy. This study adds important information to PD pathogenesis and emphasizes the potential pathophysiological link between gut microbiota/metabolites and PD motor subtypes.

Genetic variants of UDP-glucuronosyltransferases 1A1, 1A6, and 1A9 in cynomolgus and rhesus macaques

1. In the cynomolgus macaque, UDP-glucuronosyltransferases (UGTs) 1As have similar molecular and enzymatic characteristics to those of their human orthologs. However, genetic polymorphisms in major cynomolgus UGT1A1/6/9 have not been investigated. 2. We re-sequenced UGT1A1, UGT1A6, and UGT1A9 in 186 cynomolgus macaques (bred in Cambodia, China, or Indonesia) and 54 rhesus macaques and found 15, 13, and 26 non-synonymous variants, respectively. 3. Of these UGT1A1, UGT1A6, and UGT1A9 variants, respectively, 10, 9, and 12 were unique to cynomolgus macaques; 4, 1, and 2 were unique to rhesus macaques; and 1, 2, and 5 were found in both cynomolgus and rhesus macaques. The frequency of the UGT1A1 mutation G69R was 23%, 28%, and 63% in cynomolgus macaques bred in Cambodia, China, and Indonesia, respectively, and 97% in rhesus macaques. 4. The O-glucuronidation activities of liver microsomes from cynomolgus and rhesus macaques with respect to estradiol, serotonin, and propofol were measured. Among these activities, liver microsomes from cynomolgus macaques heterozygous for UGT1A1 G69R (n = 11) showed significantly reduced estradiol 3-O-glucuronidation activities compared with those from wild-type animals (n = 38). 5. These results suggest genetic variants such as UGT1A1 G69R could influence the UGT1A1-mediated glucuronidation of drugs in cynomolgus and rhesus macaques.

Mechanistic characterization of UDP-glucuronic acid 4-epimerase

UDP-glucuronic acid (UDP-GlcA) is a central precursor in sugar nucleotide biosynthesis and common substrate for C4-epimerases and decarboxylases releasing UDP-galacturonic acid (UDP-GalA) and UDP-pentose products, respectively. Despite the different reactions catalyzed, the enzymes are believed to share mechanistic analogy rooted in their joint membership to the short-chain dehydrogenase/reductase (SDR) protein superfamily: Oxidation at the substrate C4 by enzyme-bound NAD⁺ initiates the catalytic pathway. Here, we present mechanistic characterization of the C4-epimerization of UDP-GlcA, which in comparison with the corresponding decarboxylation has been largely unexplored. The UDP-GlcA 4-epimerase from Bacillus cereus functions as a homodimer and contains one NAD⁺ /subunit (k_cat  = 0.25 ± 0.01 s^-1 ). The epimerization of UDP-GlcA proceeds via hydride transfer from and to the substrate's C4 while retaining the enzyme-bound cofactor in its oxidized form (≥ 97%) at steady state and without trace of decarboxylation. The k_cat for UDP-GlcA conversion shows a kinetic isotope effect of 2.0 (±0.1) derived from substrate deuteration at C4. The proposed enzymatic mechanism involves a transient UDP-4-keto-hexose-uronic acid intermediate whose formation is rate-limiting overall, and is governed by a conformational step before hydride abstraction from UDP-GlcA. Precise positioning of the substrate in a kinetically slow binding step may be important for the epimerase to establish stereo-electronic constraints in which decarboxylation of the labile β-keto acid species is prevented effectively. Mutagenesis and pH studies implicate the conserved Tyr149 as the catalytic base for substrate oxidation and show its involvement in the substrate positioning step. Collectively, this study suggests that based on overall mechanistic analogy, stereo-electronic control may be a distinguishing feature of catalysis by SDR-type epimerases and decarboxylases active on UDP-GlcA.

Crystallographic snapshots of UDP-glucuronic acid 4-epimerase ligand binding, rotation, and reduction

UDP-glucuronic acid is converted to UDP-galacturonic acid en route to a variety of sugar-containing metabolites. This reaction is performed by a NAD⁺-dependent epimerase belonging to the short-chain dehydrogenase/reductase family. We present several high-resolution crystal structures of the UDP-glucuronic acid epimerase from Bacillus cereus The geometry of the substrate-NAD⁺ interactions is finely arranged to promote hydride transfer. The exquisite complementarity between glucuronic acid and its binding site is highlighted by the observation that the unligated cavity is occupied by a cluster of ordered waters whose positions overlap the polar groups of the sugar substrate. Co-crystallization experiments led to a structure where substrate- and product-bound enzymes coexist within the same crystal. This equilibrium structure reveals the basis for a "swing and flip" rotation of the pro-chiral 4-keto-hexose-uronic acid intermediate that results from glucuronic acid oxidation, placing the C4' atom in position for receiving a hydride ion on the opposite side of the sugar ring. The product-bound active site is almost identical to that of the substrate-bound structure and satisfies all hydrogen-bonding requirements of the ligand. The structure of the apoenzyme together with the kinetic isotope effect and mutagenesis experiments further outlines a few flexible loops that exist in discrete conformations, imparting structural malleability required for ligand rotation while avoiding leakage of the catalytic intermediate and/or side reactions. These data highlight the double nature of the enzymatic mechanism: the active site features a high degree of precision in substrate recognition combined with the flexibility required for intermediate rotation.

Molecular characterization of UDP-glucuronosyltransferases 3A and 8A in cynomolgus macaques

UDP-glucuronosyltransferases (UGTs) are drug-metabolizing enzymes essential for the metabolism of endogenous substrates and xenobiotics. The cynomolgus macaque is a nonhuman primate species widely used in drug metabolism studies. The molecular characteristics of UGTs have been extensively investigated in humans, but they remain to be elucidated in cynomolgus macaques. In this study, cynomolgus macaque UGT3A1, UGT3A2, and UGT8A1 cDNAs were isolated and characterized. Amino acid sequences deduced from cynomolgus UGT3A1, UGT3A2, and UGT8A1 cDNAs were highly identical with their human orthologs (93, 96, and 99%, respectively) and were closely clustered in a phylogenetic tree. In the genome, cynomolgus UGT3A and UGT8A genes were located in the regions corresponding to those of their human orthologs. Among the 10 tissue types analyzed, expression of cynomolgus UGT3A1 and UGT3A2 mRNAs was detected in liver, kidney, and testis; the UGT3A1 and UGT3A2 mRNAs were most abundant in liver and testis, respectively. Cynomolgus UGT8A1 was most abundantly expressed in kidney, followed by brain, jejunum, and testis. These results suggest that cynomolgus UGT3As and UGT8A1 have molecular similarities to their human orthologs.

Regional distributions of UDP-glucuronosyltransferase activities toward estradiol and serotonin in the liver and small intestine of cynomolgus macaques

The cynomolgus macaque is a nonhuman primate species that is often used in drug metabolism studies during drug development. However, the localization of UDP-glucuronosyltransferases (UGTs), essential drug-metabolizing enzymes, has not been fully investigated in the liver and small intestine of cynomolgus macaques. In this study, UGT activities were analyzed in liver (five lobes) and small intestine (the duodenum and six sections from the proximal jejunum to the distal ileum) using typical probe substrates of human UGTs: 7-hydroxycoumarin, estradiol, serotonin, propofol, and zidovudine. In liver, UGT activities with respect to all substrates were detected, and the activity levels were similar in all liver lobes of the cynomolgus macaques tested. In contrast, in the small intestine, UGT activities toward all substrates were detected, but their levels generally decreased from jejunum to ileum in cynomolgus macaques. The localization of estradiol 3-O-glucuronosyltransferases and serotonin O-glucuronosyltransferases (which are mainly UGT1A enzymes) appear to be different in liver and small intestine. These results collectively suggest that, in cynomolgus macaques, UGT1As are differentially localized in the small intestine but are relatively homogeneously distributed in the liver.

Advances in engineering UDP-sugar supply for recombinant biosynthesis of glycosides in microbes

Plant glycosides are of great interest for industries. Glycosylation of plant secondary metabolites can greatly improve their solubility, biological activity, or stability. This allows some plant glycosides to be used as food additives, cosmetic products, health products, antisepsis and anti-cancer drugs. With the continuous expansion of market demand, a variety of biological fermentation technologies has emerged. This review focuses on recombinant microbial biosynthesis of plant glycosides, which uses UDP-sugars as precursors, and summarizes various strategies to increase the yield of glycosides with a key concentration on UDP-sugar supply based on four aspects, i.e., gene overexpression, UDP-sugar recycling, mixed fermentation, and carbon co-utilization. Meanwhile, the application potential and advantages of various techniques are introduced, which provide guidance to the development of high-yield strains for recombinant microbial production of plant glycosides. Finally, the technical challenges of glycoside biosynthesis are pointed out with discussions on future directions of improving the yield of recombinantly synthesized glycosides.

Compound Characterization and Metabolic Profile Elucidation after In Vitro Gastrointestinal and Hepatic Biotransformation of an Herniaria hirsuta Extract Using Unbiased Dynamic Metabolomic Data Analysis

Herniaria hirsuta L. (Caryophyllaceae) is used for treatment of urinary stones and as a diuretic. Little is known about the active compounds and the mechanism of action. The phytochemical composition of H. hirsuta was comprehensively characterized using UHPLC-UV-HRMS (Ultrahigh-Performance Liquid Chromatography-Ultraviolet-High Resolution Mass Spectrometry) data. An in vitro gastrointestinal model was used to simulate biotransformation, which allowed the monitoring of the relative abundances of individual compounds over time. To analyze the longitudinal multiclass LC-MS data, XCMS, a platform that enables online metabolomics data processing and interpretation, and EDGE, a statistical method for time series data, were used to extract significant differential profiles from the raw data. An interactive Shiny app in R was used to rate the quality of the resulting features. These ratings were used to train a random forest model. The most abundant aglycone after gastrointestinal biotransformation was subjected to hepatic biotransformation using human S9 fractions. A diversity of compounds was detected, mainly saponins and flavonoids. Besides the known saponins, 15 new saponins were tentatively identified as glycosides of medicagenic acid, acetylated medicagenic acid and zanhic acid. It is suggested that metabolites of phytochemicals present in H. hirsuta, most likely saponins, are responsible for the pharmaceutical effects. It was observed that the relative abundance of saponin aglycones increased, indicating loss of sugar moieties during colonic biotransformation, with medicagenic acid as the most abundant aglycone. Hepatic biotransformation of this aglycone resulted in different metabolites formed by phase I and II reactions.

Assessment of the inhibition risk of paris saponins, bioactive compounds from Paris polyphylla, on CYP and UGT enzymes via cocktail inhibition assays

Paris saponins, also known as polyphyllins, are natural compounds extracted from Paris polyphylla, which have many pharmacological activities, such as anti-inflammation and anti-cancer. In particular, paris saponin I, II, VII and polyphyllin VI are the components of the quality standard for Paris polyphylla. However, the inhibition risk of polyphyllins on cytochrome P450 (CYP) and UDP-glucuronosyltransferases (UGT) remains unclear. Therefore, this report investigated the potential inhibitory effects of paris saponin I, II, VII and polyphyllin VI on the activities of CYP (CYP1A2, CYP2B1, CYP2C11, CYP2D1, CYP2E1 and CYP3A2) and UGT (UGT1A1, UGT1A3, UGT1A6, PROG and AZTG) through cocktail inhibition assays in vitro. In the study of CYP, polyphyllin VI exhibited weak inhibition on CYP2D1 activity in rat liver microsomes with IC₅₀ value at 45.2 μM, while paris saponin VII weakly inhibited CYP2C11 and CYP2E1 activities with IC₅₀ value at 42.0 and 67.7 μM, respectively. In the study of UGT, none of the four steroidal saponins showed significant inhibition risk. In conclusion, paris saponin I, II, VII and polyphyllin VI have very low potential to cause the possible toxicity and drug interactions involving CYP and UGT enzymes, indicating that they are safe enough to take with drugs.

Wogonin glucuronidation in liver and intestinal microsomes of humans, monkeys, dogs, rats, and mice

Wogonin, one of the flavonoids isolated from Scutellaria baicalensis, exhibits some beneficial bioactivities, including anti-inflammatory and anticancer effects, and is metabolized into glucuronide by UDP-glucuronosyltransferase (UGT) enzymes in humans. In the present study, wogonin glucuronidation was examined in the liver and intestinal microsomes of humans, monkeys, dogs, rats, and mice using a kinetic analysis.The kinetics of wogonin glucuronidation by liver microsomes followed the biphasic model in all species examined. CL_int values (x-intercept) based on v versus V/[S] plots were rats > humans ≈ monkeys > mice > dogs. The kinetics of intestinal microsomes fit the Michaelis-Menten model for humans, monkeys, rats, and mice and the substrate inhibition model for dogs. CL_int values were rats > monkeys > mice > dogs > humans. The tissue dependence of CL_int values was liver microsomes > intestinal microsomes for humans, dogs, and rats, and liver microsomes ≈ intestinal microsomes for monkeys and mice.These results demonstrated that the metabolic abilities of UGT enzymes toward wogonin in the liver and intestines markedly differ among humans, monkeys, dogs, rats, and mice, and suggest that species differences are closely associated with the biological effects of wogonin.

New planar assay for streamlined detection and quantification of β-glucuronidase inhibitors applied to botanical extracts

The inhibition of the β-glucuronidase released from gut bacteria is associated with specific health-related benefits. Though a number of β-glucuronidase inhibition assays are currently in use, none of them can directly measure the relevant activity of each single constituent in a complex mixture, without prior separation and tedious isolation of the pure compounds. Thus, the hyphenation of the high performance thin layer chromatography (HPTLC) technique with a β-glucuronidase inhibition assay was investigated and successfully demonstrated for the first time. A colorimetric as well as fluorometric detection of the inhibitors was achieved using 5-bromo-4-chloro-3-indolyl-β-D-glucuronide as a substrate. Hence, β-glucuronidase inhibitors were detected as bright zones against an indigo blue or fluorescent background. The established method was optimized and validated employing the well-known inhibitor d-saccharic acid 1,4-lactone monohydrate. As proof of concept, the suitability of the new workflow was verified through analysis of two botanical extracts, Primula boveana and silymarin flavonolignans from Silybum marianum fruits. The found inhibitors were identified by spectroscopic methods; one of them, 3ʹ-O-(β-galactopyranosyl)-flavone, is here described as a newly isolated natural compound. The new hyphenation HPTLC-UV/Vis/FLD-β-glucuronidase inhibition assay-HRMS covers four orthogonal dimensions, i.e. separation, spectral detection, biochemical activity and structural characterization, in a highly targeted time- and material-saving workflow for analysis of complex or costly mixtures.

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Coupling of HPTLC to the β-glucuronidase inhibition assay is demonstrated.

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Colorimetric and fluorometric detection of the inhibition was given.

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A new β-glucuronidase inhibiting flavonoid in P. boveana was elucidated.

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HPTLC-HRMS analysis of other β-glucuronidase inhibitors is shown for silymarin.

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Analysis of rare plants (low extract amount) is possible with the new planar assay.

Molecular characterization of functional UDP-glucuronosyltransferases 1A and 2B in common marmosets

UDP-glucuronosyltransferases (UGTs) are essential drug-conjugation enzymes that metabolize a variety of endobiotic and xenobiotic substrates. The molecular characteristics of UGTs have been extensively investigated in humans, but remain to be investigated in common marmosets, a nonhuman primate species widely used in drug metabolism studies. In this study, 11 UGT cDNAs (UGT1A1, 1A3, 1A4, 1A6, 1A7, and 1A9; and UGT2B49, 2B50, 2B51, 2B52, and 2B53) were isolated and characterized in marmosets. Marmoset UGT1As had high sequence identities (89-93%) with human UGT1As, but the sequence identities of marmoset UGT2Bs were lower (82-86%). Marmoset UGTs were found to be phylogenetically close to human UGTs. Just as human UGT1As do, marmoset UGT1A genes shared exons 2-5 and contained a variable exon 1 unique to each gene; in contrast, marmoset UGT2B genes contained six unique exons. Moreover, marmoset and human UGT1A and UGT2B gene clusters were located in corresponding regions in their respective genomes. Among the five tissue types tested, marmoset UGT mRNAs were most abundantly expressed in liver, jejunum, and/or kidney, i.e., in tissues important for drug metabolism, just as human UGTs are. Among the 11 marmoset UGT mRNAs investigated, marmoset UGT1A9, 1A4, and 1A6 mRNAs were the most abundantly expressed in liver, small intestine, and kidney, respectively. Marmoset liver microsomes and recombinant UGT1A proteins catalyzed the glucuronidation of the same substrates that human UGT1As catalyze, including estradiol, trifluoperazine, 4-methylumbelliferone, serotonin, 4-nitrophenol, and propofol. Trifluoperazine was glucuronidated by marmoset liver microsomes, but not by any of the UGT1A isoforms examined under the present conditions. These results collectively suggest that functional marmoset UGTs have generally similar molecular characteristics to human UGTs.

Herb-Drug Interactions and Hepatotoxicity

BACKGROUND

In recent times, herbals or phytomedicines have become very popular due to their global acceptance as a complementary and alternative remedy. While modern drugs are commercially available only after laboratory validations, clinical trials, as well as approval from drug regulatory authorities, majority of the marketed herbal products lack such scientific evidence of efficacy and safety. This results in herb or herb-drug interaction induced unfavorable clinical outcomes without crucial documentation on their temporal relations and concomitant use.

METHODS

An online literature search for peer-reviewed articles was conducted on the PubMed, Europe PMC, Medline and Google Scholar portals, using the phrases: complementary & alternative medicine, traditional Chinese medicine, herb-drug interaction, mechanisms of herb-drug interaction, herb-induced toxicity, herbal hepatotoxicity and causality, traditional medicine, viral hepatitis, etc.

Results

The retrieved data showed that globally, patients are attracted to herbal remedies with the misconception that these are completely safe and therefore, use them simultaneously with prescription drugs. Notably, there exists a potential risk of herb-drug interactions leading to some adverse side effects, including hepatotoxicity. The toxicological effect of a drug or herb is due to the inhibition of drug metabolizing enzymes (e.g., cytochrome P450), including interactions with certain prescription drugs through various mechanisms. Several cases of hepatotoxicity due to use of herbals in viral hepatitis-related liver diseases have been recently reported. However, limited experimental data and clinical evidence on herbal pharmacokinetics hamper the evaluation and reporting of adverse reactions and the underlying mechanisms.

CONCLUSION

Herb-drug interaction related morbidity is thus an emerging serious public health issue with broad implications for clinicians, pharmaceutical industries and health authorities. Nonetheless, despite increasing recognition of herb-drug interaction, a standard system for interaction prediction and evaluation is still nonexistent. This review article discusses the herb-drug interactions related hepatotoxicity and underlying mechanisms, including drug metabolizing enzymes and their regulation.

Functional and molecular characterization of UDP-glucuronosyltransferase 2 family in cynomolgus macaques

UDP-glucuronosyltransferases (UGTs) are essential enzymes metabolizing endogenous and exogenous chemicals. However, characteristics of UGTs have not been fully investigated in molecular levels of cynomolgus macaques, one of non-human primates widely used in preclinical drug metabolism studies. In this study, three UGT2A cDNAs (UGT2A1, 2A2, and 2A3) were isolated and characterized along with seven UGT2Bs previously identified in cynomolgus macaques. Several transcript variants were found in cynomolgus UGT2A1 and UGT2A2, like human orthologs. Cynomolgus UGT2A and UGT2B amino acid sequences were highly identical (87-96%) to their human counterparts. By phylogenetic analysis, all these cynomolgus UGT2s were more closely clustered with their human homologs than with dog, rat, or mouse UGT2s. Especially, UGT2As showed orthologous relationships between humans and cynomolgus macaques. All the cynomolgus UGT2 mRNAs were expressed in livers, jejunum, and/or kidneys abundantly, except that UGT2A1 and UGT2A2 mRNAs were predominantly expressed in nasal mucosa, like human UGT2s. UGT2A and UGT2B genes together form a gene cluster in the cynomolgus and human genome. Among the seven cynomolgus UGT2Bs heterologously expressed in yeast, UGT2B9 and UGT2B30 showed activities in estradiol 17-O-glucuronidation and morphine 3-O-glucuronidation but did not show activities in estradiol 3-O-glucuronidation, similar to human UGT2Bs. In liver microsomes, cynomolgus macaques showed higher estradiol 17-O-glucuronidase and morphine 3-O-glucuronidase activities than humans, suggesting functional activities of the responsible UGT2B enzymes in cynomolgus macaques. Therefore, cynomolgus UGT2s had overall molecular similarities to human UGT2s, but also showed some differences in UGT2B enzyme properties.

Hysteresis and Allostery in Human UDP-Glucose Dehydrogenase Require a Flexible Protein Core

Human UDP-glucose dehydrogenase (hUGDH) oxidizes UDP-glucose to UDP-glucuronic acid, an essential substrate in the phase II metabolism of drugs. The activity of hUGDH is regulated by the conformation of a buried allosteric switch (T131 loop/α6 helix). Substrate binding induces the allosteric switch to slowly isomerize from an inactive E* conformation to the active E state, which can be observed as enzyme hysteresis. When the feedback inhibitor UDP-xylose binds, the allosteric switch and surrounding residues in the protein core repack, converting the hexamer into an inactive, horseshoe-shaped complex (EΩ). This allosteric transition is facilitated by large cavities and declivities in the protein core that provide the space required to accommodate the alternate packing arrangements. Here, we have used the A104L substitution to fill a cavity in the E state and sterically prevent repacking of the core into the EΩ state. Steady state analysis shows that hUGDHA104L binds UDP-xylose with lower affinity and that the inhibition is no longer cooperative. This means that the allosteric transition to the high-UDP-xylose affinity EΩ state is blocked by the substitution. The crystal structures of hUGDHA104L show that the allosteric switch still adopts the E and E* states, albeit with a more rigid protein core. However, the progress curves of hUGDHA104L do not show hysteresis, which suggests that the E* and E states are now in rapid equilibrium. Our data suggest that hysteresis in native hUGDH originates from the conformational entropy of the E* state protein core.

Age-related changes in hepatic expression and activity of drug metabolizing enzymes in male wild-type and breast cancer resistance protein knockout mice

This study aimed to reveal age-related changes in the expression and activity of seven hepatic drug metabolizing enzymes (DMEs) in male wild-type and breast cancer resistance protein knockout (Bcrp1^-/- ) FVB mice. The protein expression of four cytochrome P450 (Cyps) (Cyp3a11, 2d22, 2e1, and 1a2), and three UDP-glucuronosyltransferases (Ugts) (Ugt1a1, 1a6a, and 1a9) in liver microsomes of wild-type and Bcrp1^-/- FVB mice at different ages were determined using a validated ultra high performance liquid chromatography with tandem mass spectrometry (UHPLC-MS/MS) method. The activities and mRNA levels of these DMEs were measured using the probe substrates method and real-time PCR, respectively. In the liver of wild-type FVB mice, Cyp3a11, 2d22, 2e1, 1a2, Ugt1a1, and 1a6a displayed maximum protein levels at 6-9 weeks of age. Cyp1a2, Ugt1a1, 1a6a, and 1a9 showed maximum activities at 6-9 weeks of age, whereas Cyp3a11, 2d22, and 2e1 showed maximum activities in 1-3-week-old mice. Additionally, most of the DMEs showed maximum mRNA levels in 17-week-old mice liver. Compared with wild-type FVB mice, the protein levels of these DMEs showed no significant changes in Bcrp1^-/- FVB mice liver. However, the activity of Cyp2e1 was increased and that of Cyp2d22 was decreased. In conclusion, the seven hepatic DMEs in FVB mice liver showed significant alterations in an isoform-specific manner with increased age. Although the protein levels of these DMEs showed no significant changes, the activities of Cyp2e1 and 2d22 were changed in Bcrp1^-/- mice.

β-Glucuronidases of opportunistic bacteria are the major contributors to xenobiotic-induced toxicity in the gut

Gut bacterial β-D-glucuronidases (GUSs) catalyze the removal of glucuronic acid from liver-produced β-D-glucuronides. These reactions can have deleterious consequences when they reverse xenobiotic metabolism. The human gut contains hundreds of GUSs of variable sequences and structures. To understand how any particular bacterial GUS(s) contributes to global GUS activity and affects human health, the individual substrate preference(s) must be known. Herein, we report that representative GUSs vary in their ability to produce various xenobiotics from their respective glucuronides. To attempt to explain the distinct substrate preference, we solved the structure of a bacterial GUS complexed with coumarin-3-β-D-glucuronide. Comparisons of this structure with other GUS structures identified differences in loop 3 (or the α2-helix loop) and loop 5 at the aglycone-binding site, where differences in their conformations, hydrophobicities and flexibilities appear to underlie the distinct substrate preference(s) of the GUSs. Additional sequence, structural and functional analysis indicated that several groups of functionally related gut bacterial GUSs exist. Our results pinpoint opportunistic gut bacterial GUSs as those that cause xenobiotic-induced toxicity. We propose a structure-activity relationship that should allow both the prediction of the functional roles of GUSs and the design of selective inhibitors.

Human Enzymes for Organic Synthesis

Human enzymes have been widely studied in various disciplines. The number of reactions taking place in the human body is vast, and so is the number of potential catalysts for synthesis. Herein, we focus on the application of human enzymes that catalyze chemical reactions in course of the metabolism of drugs and xenobiotics. Some of these reactions have been explored on the preparative scale. The major field of application of human enzymes is currently drug development, where they are applied for the synthesis of drug metabolites.

Acyl Glucuronide Metabolites of 6-Chloro-5-[4-(1-hydroxycyclobutyl)phenyl]-1 H-indole-3-carboxylic Acid (PF-06409577) and Related Indole-3-carboxylic Acid Derivatives are Direct Activators of Adenosine Monophosphate-Activated Protein Kinase (AMPK)

Studies on indole-3-carboxylic acid derivatives as direct activators of human adenosine monophosphate-activated protein kinase (AMPK) α1β1γ1 isoform have culminated in the identification of PF-06409577 (1), PF-06885249 (2), and PF-06679142 (3) as potential clinical candidates. Compounds 1-3 are primarily cleared in animals and humans via glucuronidation. Herein, we describe the biosynthetic preparation, purification, and structural characterization of the glucuronide conjugates of 1-3. Spectral characterization of the purified glucuronides M1, M2, and M3 indicated that they were acyl glucuronide derivatives. In vitro pharmacological evaluation revealed that all three acyl glucuronides retained selective activation of β1-containing AMPK isoforms. Inhibition of de novo lipogenesis with representative parent carboxylic acids and their respective acyl glucuronide conjugates in human hepatocytes demonstrated their propensity to activate cellular AMPK. Cocrystallization of the AMPK α1β1γ1 isoform with 1-3 and M1-M3 provided molecular insights into the structural basis for AMPK activation by the glucuronide conjugates.

Design and optimization of the cocktail assay for rapid assessment of the activity of UGT enzymes in human and rat liver microsomes

Along with the prevalence of drug combination therapies, an increasing number of cases about drug-drug interactions (DDI) have been reported, which has drawn a lot of attention due to the potential toxicity and/or therapeutic failure. Pharmacokinetic interactions based on drug metabolic enzymes should be responsible for a great many of DDI. UDP-glucuronosyltransferases (UGT) as the main phase II metabolic enzymes are involved in the metabolism of many endogenous and exogenous substrates. Herein, we designed and optimized a validated cocktail method for the simultaneous evaluation of drug-mediated inhibition of the main five UGT isoforms using respective specific probe substrates (estradiol for UGT1A1, chenodeoxycholic acid for UGT1A3, serotonin for UGT1A6, propofol for UGT1A9/PROG and zidovudine for UGT2B7/AZTG) in human and rat liver microsomes by liquid chromatography-tandem mass spectrometry (LCMS/MS). Moreover, we investigated the risk of interactions among UGT probe substrates, and validated the cocktail method by known positive inhibitors of UGT isoforms. To minimize the substrates interaction, we developed two cocktail subgroups which were further optimized via exploring the experimental conditions. In particular, the cocktail inhibition assay for rapid assessment of in vitro rat UGTs was firstly reported and the values of K_m in the liver microsomes from humans and rats were close to each other in the specific UGT subtype. In conclusion, this study has successfully established the cocktail approach to explore UGT activity, especially for UGT inhibition in a fast and efficient way.

Catechol-O-Methyltransferase and UDP-Glucuronosyltransferases in the Metabolism of Baicalein in Different Species

BACKGROUND

Baicalein is the major bioactive flavonoid in some herb medicines and dietary plants; however, the detailed metabolism pathway of its major metabolite oroxylin A-7-O-β-D-glucuronide in human was not clear. It was important to illustrate the major metabolic enzymes that participate in its elimination for the clinic use of baicalein.

OBJECTIVES

We first revealed a two-step metabolism profile for baicalein and illustrated the combination of catechol-O-methyltransferase (COMT) and uridine diphosphate-glucuronosyltransferases (UGTs) in drug metabolism, further evaluated its bioactivity variation during drug metabolism.

METHODS

The metabolism profiles were systematically characterized in different human biology preparations; after then, the anti-inflammatory activities of metabolites were evaluated in LPS-induced RAW264.7 cell.

RESULTS

The first-step metabolite of baicalein was isolated and identified as oroxylin A; soluble-bound COMT (S-COMT) was the major enzyme responsible for its biotransformation. Specially, position 108 mutation of S-COMT significantly decreases the elimination. Meantime, oroxylin A was rapidly metabolized by UGTs, UGT1A1, -1A3, -1A6, -1A7, -1A8, -1A9, and -1A10 which were involved in the glucuronidation. Considerable species differences were observed with 1060-fold K _m (3.05 ± 1.86-3234 ± 475 μM) and 330-fold CL_int (5.93-1973 μL/min/mg) variations for baicalein metabolism. Finally, the middle metabolite oroxylin A exhibited a potent anti-inflammatory activity with the IC₅₀ value of 28 μM.

CONCLUSION

The detailed kinetic parameters indicated that COMT provide convenience for the next glucuronidation; monkey would be a preferred animal model for the preclinical investigation of baicalein. Importantly, oroxylin A should be reconsidered in evaluating baicalein efficacy against inflammatory diseases.

Chemo-Enzymatic Synthesis, Structural and Stereochemical Characterization, and Intrinsic Degradation Kinetics of Diastereomers of 1-β-O-Acyl Glucuronides Derived from Racemic 2-{4-[(2-Methylprop-2-en-1-yl)amino]phenyl}propanoic Acid

Alminoprofen, (RS)-2-{4-[(2-methylprop-2-en-1-yl)amino]phenyl}propanoic acid (ALP) 1, is a racemic drug categorized as a 2-arylpropanoic acid-class nonsteroidal anti-inflammatory drug. Pharmacokinetic studies of 1 in patients have revealed that the corresponding acyl glucuronide 5 is a major urinary metabolite, but little is known about the structure and stereochemistry of 5. The present work describes the synthesis of a diastereomeric mixture of 1-β-O-acyl glucuronides (2RS)-5 from 1 and methyl 2,3,4-tri-O-acetyl-1-bromo-1-deoxy-α-d-glucopyranuronate 2 using our chemo-enzymatic method that has complete specificity for the β-configuration. The structure of (2RS)-5 was characterized by ¹H and ¹³C NMR spectroscopy and high-resolution mass spectrometry as well as by complete hydrolysis by β-glucuronidase. The absolute stereochemistry of (2RS)-5 was determined by comparison with (2R)-5 synthesized alternatively from (2R)-1 and 2. Compound (2R)-1 was prepared in two steps starting from chiral (R)-2-(4-nitrophenyl)propanoic acid (2R)-6. Chiral resolution of (2RS)-1 was achieved using a chiral high-performance liquid chromatography column, and its stereochemistry was determined by comparison with (2R)-1. The intrinsic degradation rate constant of (2R)-5 was 0.405 ± 0.002 h^-1, which is approximately twice that of (2S)-5 (the k value was 0.226 ± 0.002 h^-1) under physiological conditions (pH 7.40, 37 °C).

Pharmacokinetic-pharmacodynamic correlations in the development of ginger extract as an anticancer agent

Anticancer efficacy of ginger phenolics (GPs) has been demonstrated in various in vitro assays and xenograft mouse models. However, only sub-therapeutic plasma concentrations of GPs were detected in human and mouse pharmacokinetic (PK) studies. Intriguingly, a significant portion of GPs occurred as phase II metabolites (mainly glucuronide conjugates) in plasma. To evaluate the disposition of GPs and understand the real players responsible for efficacy, we performed a PK and tissue distribution study in mice. Plasma exposure of GPs was similar on day 1 and 7, suggesting no induction or inhibition of clearance pathways. Both free and conjugated GPs accumulated in all tissues including tumors. While non-cytotoxicity of 6-ginerol glucuronide precluded the role of conjugated GPs in cell death, the free forms were cytotoxic against prostate cancer cells. The efficacy of ginger was best explained by the reconversion of conjugated GPs to free forms by β-glucuronidase, which is over-expressed in the tumor tissue. This previously unrecognized two-step process suggests an instantaneous conversion of ingested free GPs into conjugated forms, followed by their subsequent absorption into systemic circulation and reconversion into free forms. This proposed model uncovers the mechanistic underpinnings of ginger's anticancer activity despite sub-therapeutic levels of free GPs in the plasma.

Diversion of a thioglycoligase for the synthesis of 1-O-acyl arabinofuranoses

An arabinofuranosylhydrolase from the GH51 family was transformed into an acyl transferase by mutation of the catalytic acid/base amino acid.