Metabolism of (+)- and (-)-menthols by CYP2A6 in human liver microsomes

Exogenous Volatile Organic Compound (EVOC®) Breath Testing Maximizes Classification Performance for Subjects with Cirrhosis and Reveals Signs of Portal Hypertension

Background: Cirrhosis detection in primary care relies on low-performing biomarkers. Consequently, up to 75% of subjects with cirrhosis receive their first diagnosis with decompensation when causal treatments are less effective at preserving liver function. We investigated an unprecedented approach to cirrhosis detection based on dynamic breath testing. Methods: We enrolled 29 subjects with cirrhosis (Child-Pugh A and B), and 29 controls. All subjects fasted overnight. Breath samples were taken using Breath Biopsy® before and at different time points after the administration of 100 mg limonene. Absolute limonene breath levels were measured using gas chromatography-mass spectrometry. Results: All subjects showed a >100-fold limonene spike in breath after administration compared to baseline. Limonene breath kinetics showed first-order decay in >90% of the participants, with higher bioavailability in the cirrhosis group. At the Youden index, baseline limonene levels showed classification performance with an area under the roc curve (AUROC) of 0.83 ± 0.012, sensitivity of 0.66 ± 0.09, and specificity of 0.83 ± 0.07. The best performing timepoint post-administration was 60 min, with an AUROC of 0.91, sensitivity of 0.83 ± 0.07, and specificity of 0.9 ± 0.06. In the cirrhosis group, limonene bioavailability showed a correlation with MELD and fibrosis indicators, and was associated with signs of portal hypertension. Conclusions: Dynamic limonene breath testing enhances diagnostic performance for cirrhosis compared to static testing. The correlation with disease severity suggests potential for monitoring therapeutic interventions. Given the non-invasive nature of breath collection, a dynamic limonene breath test could be implemented in primary care.

Using Cg05575921 methylation to predict lung cancer risk: a potentially bias-free precision epigenetics approach

The decision to engage in lung cancer screening (LCS) necessitates weighing benefits versus harms. Previously, clinicians in the United States have used the PLCOM2012 algorithm to guide LCS decision-making. However, that formula contains race and gender-based variables. Previously, using data from a European study, Bojesen and colleagues have suggested that cg05575921 methylation could guide decision-making. To test this hypothesis in a more diverse American population, we examined DNA and clinical data from 3081 subjects from the National Lung Screening Trial (NLST) study. Using survival analysis, we found a simple linear predictor consisting of age, pack-year consumption and cg05575921, to have the best predictive power among several alternatives (AUC = 0.66). Results showed that the highest quartile of risk was more than 2-fold more likely to develop lung cancer than those in the lowest quartile. Race, ethnicity, and gender had no effect on prediction with both cg05575921 and pack years contributing equally (both p < 0.003) to risk prediction. Current smokers had considerably lower methylation than former smokers (46% vs 67%; p < 0.001) with the average methylation of those who quit approaching 80% after 25 years of cessation. Finally, current male smokers had lower mean cg05575921 percentage than female smokers (46% vs 49%; p < 0.001). We conclude that cg05575921 (along with age and pack years) can be used to guide LCS decision-making, and additional studies might focus on how best to use methylation to inform decision-making.

Diastereoselective metabolism of homomenthyl salicylate (homosalate): Identification of relevant human exposure biomarkers

Homosalate (HMS) is a salicylate UV filter broadly used in sunscreens and personal care products. The aim of this study was the collection of human toxicokinetic data on HMS as a tool for risk assessment. For this purpose, metabolism and urinary excretion after a single oral HMS dose (98.2-149.1 µg (kg body weight)^-1) were investigated in four volunteers (two male, two female). As commercial products generally contain a mixture of cis- and trans-HMS, both cis-rich and trans-rich isomer mixtures were studied to investigate possible differences in metabolism. Initial metabolite screening tentatively identified six oxidative metabolite subgroups, of which hydroxylated and carboxylic acid metabolites were studied in more detail. Unchanged parent HMS and the previously identified HMS metabolites 5-((2-hydroxybenzoyl)oxy)-3,3-dimethylcyclohexane-1-carboxylic acid (HMS-CA) and 3-hydroxy-3,5,5-trimethylcyclohexyl 2-hydroxybenzoate (3OH-HMS), respectively, were quantified separately as cis- and trans-isomers via authentic standards by isotope dilution analysis. In addition, further alkyl-hydroxylated and carboxylic acid metabolites were investigated semi-quantitatively. Peak concentrations in urine were reached 1.5-6.3 h post-dose and more than 80 % of each of the quantitatively investigated metabolites (and at least 70 % of the semi-quantitatively investigated metabolites) was excreted within the first 24 h. Plasma and urine data indicated that oral bioavailability of cis-HMS was one order of magnitude below that of trans-HMS. Furthermore, the mean total urinary excretion fraction (F_ue) for the metabolites derived from trans-HMS (6.4 %) was two orders of magnitude higher than for the metabolites derived from cis-HMS (0.045 %). Our data proves diastereoselectivity in toxicokinetics of cis- and trans-HMS, emphasizing the necessity to address isomer ratios in future studies including HMS exposure and risk assessments.

Randomised trial: Peppermint oil (menthol) pharmacokinetics in children and effects on gut motility in children with functional abdominal pain

AIMS

Little is known regarding the pharmacokinetics and pharmacodynamics of menthol, the active ingredient in peppermint oil (PMO). Our aim was to investigate the pharmacokinetics of menthol at 3 dose levels in children and determine their effects on gut motility and transit.

METHODS

Thirty children ages 7-12 years with functional abdominal pain underwent wireless motility capsule (WMC) testing. Approximately 1 week later they were randomized to 180, 360 or 540 mg of enteric coated PMO (10 participants per dose). Menthol pharmacokinetics were determined via blood sampling over 24 hours. They then took their respective dose of PMO (180 mg once, 180 mg twice or 180 mg thrice daily) for 1 week during which time the WMC test was repeated.

RESULTS

Evaluable area under the plasma concentration vs. time curve (AUC_last ) data were available in 29 of 30 participants. A direct linear relationship (apparent dose-proportionality for systemic menthol exposure) was observed between PMO dose and menthol systemic exposure with mean elimination half-life 2.1, 3.5 and 4.6 hours for the 180, 360 and 540 mg doses, respectively. WMC technical issues precluded complete motility data in all participants. Colonic transit time was inversely related to AUC_last (P = .003); transit time in other regions was not affected. In contrast, stomach, small bowel and whole gut (but not colonic) contractility positively correlated with menthol AUC_last (P < .05).

CONCLUSION

Pharmacokinetics and pharmacodynamics of menthol derived from PMO demonstrated apparent dose-proportionality. A higher dose of PMO may be needed to achieve maximal gut response. www.clinicaltrials.gov NCT03295747.

Pharmacological interventions for pediatric irritable bowel syndrome

INTRODUCTION

Irritable bowel syndrome is a common functional gastrointestinal disorder in children, characterized by recurrent abdominal pain associated with altered bowel habits in terms of both frequency and consistency. According to change in stool consistency it is categorized into 4 subtypes. From the etiological perspective, it is a combination of factors takes part in symptoms' generation, the overall treatment response rate is often unsatisfactory if a multidisciplinary is not pursued.

AREAS COVERED

The aim of this manuscript is to summarize the current pharmacotherapy in pediatric irritable bowel syndrome in order to aid clinicians in treating this challenging disorder.

EXPERT OPINION

Most evidence involving pediatric populations rely on open label or retrospective studies and/or are not specifically designed for irritable bowel syndrome but tend to generalize their results to mixed populations of children with functional gastrointestinal disorders. A high placebo response rate combined with poor patients' selection could account for the overall weak evidence supporting the use of pharmacological agents in pediatric irritable bowel syndrome. Given the multifaceted nature of the disorder, multidisciplinary approaches combining pharmacotherapy with alternative treatments is highly recommendable.

Impact of Menthol on Growth and Photosynthetic Function of Breviolum Minutum (Dinoflagellata, Dinophyceae, Symbiodiniaceae) and Interactions with its Aiptasia Host

Environmental change, including global warming and chemical pollution, can compromise cnidarian-(e.g., coral-) dinoflagellate symbioses and cause coral bleaching. Understanding the mechanisms that regulate these symbioses will inform strategies for sustaining healthy coral-reef communities. A model system for corals is the symbiosis between the sea anemone Exaiptasia pallida (common name Aiptasia) and its dinoflagellate partners (family Symbiodiniaceae). To complement existing studies of the interactions between these organisms, we examined the impact of menthol, a reagent often used to render cnidarians aposymbiotic, on the dinoflagellate Breviolum minutum, both in culture and in hospite. In both environments, the growth and photosynthesis of this alga were compromised at either 100 or 300 µM menthol. We observed reduction in PSII and PSI functions, the abundances of reaction-center proteins, and, at 300 µM menthol, of total cellular proteins. Interestingly, for free-living algae exposed to 100 µM menthol, an initial decline in growth, photosynthetic activities, pigmentation, and protein abundances reversed after 5-15 d, eventually approaching control levels. This behavior was observed in cells maintained in continuous light, but not in cells experiencing a light-dark regimen, suggesting that B. minutum can detoxify menthol or acclimate and repair damaged photosynthetic complexes in a light- and/or energy-dependent manner. Extended exposures of cultured algae to 300 µM menthol ultimately resulted in algal death. Most symbiotic anemones were also unable to survive this menthol concentration for 30 d. Additionally, cells impaired for photosynthesis by pre-treatment with 300 µM menthol exhibited reduced efficiency in re-populating the anemone host.

The Biological Impact of Menthol on Tobacco Dependence

In the 1920s, tobacco companies created a marketing campaign for what would one day be their most profitable series of products: mentholated tobacco cigarettes. Menthol provides the smoker with a pleasant mint flavor in addition to a cooling sensation of the mouth, throat, and lungs, giving relief from the painful irritation caused by tobacco smoke. Promising a healthier cigarette using pictures of doctors in white coats and even cartoon penguins, tobacco companies promoted these cigarettes to young, beginner smokers and those with respiratory health concerns. Today, smoking tobacco cigarettes causes one in five US Americans to die prematurely, crowning it as the leading cause of preventable death. In contrast to the dubious health claims by tobacco companies, mentholated cigarettes are in fact more addictive. Smokers of mentholated cigarettes have lower successful quit rates and in some cases are resistant to both behavioral and pharmacological treatment strategies. There is now considerable evidence, especially in the last 5 years, that suggest menthol might influence the addictive potential of nicotine-containing tobacco products via biological mechanisms. First, menthol alters the expression, stoichiometry, and function of nicotinic receptors. Second, menthol's chemosensory properties operate to mask aversive properties of using tobacco products. Third, menthol's chemosensory properties aid in serving as a conditioned cue that can both enhance nicotine intake and drive relapse. Fourth, menthol alters nicotine metabolism, increasing its bioavailability. This review discusses emerging evidence for these mechanisms, with an emphasis on preclinical findings that may shed light on why menthol smokers exhibit greater dependence.

IMPLICATIONS

Mentholated cigarettes have been shown to have greater addictive potential than their nonmentholated counterparts. Evidence is pointing toward multiple mechanisms of action by which menthol may alter tobacco dependence. Understanding menthol's biological functions as it pertains to nicotine dependence will be helpful in crafting novel pharmacotherapies that might better serve menthol smokers. In addition, a better understanding of menthol's pharmacology as it relates to tobacco dependence will be valuable for informing policy decisions on the regulation of mentholated cigarettes.

Current Knowledge on the Vascular Effects of Menthol

Menthol is a monoterpene alcohol, widely used in several food and healthcare products for its particular odor and flavor. For some decades, menthol has been known to act on the vasculature directly in the endothelium and vascular smooth muscle, with recent studies showing that it also evokes an indirect vascular response via sensory fibers. The mechanisms underlying menthol's vascular action are complex due to the diversity of cellular targets, to the interplay between signaling pathways and to the variability in terms of response. Menthol can evoke either a perfusion increase or decrease in vivo in different vascular territories, an observation that warrants a critical discussion. Menthol vascular actions in vivo seem to depend on whether the vascular territory under analysis has been directly provoked with menthol or is located deep/distant to the application site. Menthol increases perfusion of directly provoked skin regions due to a complex interplay of increased nitric oxide (NO), endothelium-derived hyperpolarization factors (EDHFs) and sensory nerve responses. In non-provoked vascular beds menthol decreases perfusion which might be attributed to heat-conservation sympathetically-mediated vasoconstriction, although an increase in tissue evaporative heat loss due the formulation ethanol may also play a role. There is increasing evidence that several of menthol's cellular targets are involved in cardiovascular diseases, such as hypertension. Thus menthol and pharmacologically-similar drugs can play important preventive and therapeutic roles, which merits further investigation.

FEMA GRAS assessment of natural flavor complexes: Mint, buchu, dill and caraway derived flavoring ingredients

In 2015, the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA) initiated a re-evaluation of the safety of over 250 natural flavor complexes (NFCs) used as flavor ingredients. NFC flavor materials include a variety of essential oils and botanical extracts. The re-evaluation of NFCs is conducted based on a constituent-based procedure outlined in 2005 and updated in 2018 that evaluates the safety of NFCs for their intended use as flavor ingredients. This procedure is applied in the re-evaluation of the generally recognized as safe (GRAS) status of NFCs with constituent profiles that are dominated by alicyclic ketones such as menthone and carvone, secondary alcohols such as menthol and carveol, and related compounds. The FEMA Expert Panel affirmed the GRAS status of Peppermint Oil (FEMA 2848), Spearmint Oil (FEMA 3032), Spearmint Extract (FEMA 3031), Cornmint Oil (FEMA 4219), Erospicata Oil (FEMA 4777), Curly Mint Oil (FEMA 4778), Pennyroyal Oil (FEMA 2839), Buchu Leaves Oil (FEMA 2169), Caraway Oil (FEMA 2238) and Dill Oil (FEMA 2383) and determined FEMA GRAS status for Buchu Leaves Extract (FEMA 4923), Peppermint Oil, Terpeneless (FEMA 4924) and Spearmint Oil, Terpeneless (FEMA 4925).

An in silico investigation of menthol metabolism

Prevalence of mentholated products for consumption has brought great importance to studies on menthol’s metabolic pathways to ensure safety, design more potent derivatives, and identify therapeutic benefits. Proposed pathways of (-)-menthol metabolism based on metabolites found experimentally in previous works by Yamaguchi, Caldwell & Farmer, Madyastha & Srivatsan and Hiki et al. were not in agreement. This in silico approach is based on the three in vivo studies and aims to resolve the discrepancies. Reactions in the pathways are conjugation with glucuronic acid/sulfate, oxidation to alcohol, aldehyde & carboxylic acid, and formation of a four-membered/five-membered ring. Gas-phase structures, standard Gibbs energies and SMD solvation energies at B3LYP/6-311++G(d,p) level were obtained for 102 compounds in the pathways. This study provides a more complete picture of menthol metabolism by combining information from three experimental studies and filling missing links in previously published pathways.

Enhanced anti-cancer activity by menthol in HepG2 cells exposed to paclitaxel and vincristine: possible involvement of CYP3A4 downregulation

Background Menthol is widely used as a constituent of functional foods and chemical drugs. In the present study, we investigated changes in the expression of cytochrome P450 isoform CYP3A4mRNA after treating human hepatocellular carcinoma HepG2 cells with menthol. We also examined the effects of pretreatment with menthol on the cytotoxic activity of paclitaxel (PAC) and vincristine (VIN), which are substrates of CYP3A4, in the cells. Methods HepG2 cells were maintained in Dulbecco's Modified Eagle's Medium. Expression of CYP3A4 was examined by the real-time polymerase chain reaction. Survival rate of HepG2 cells was evaluated by the MTT assay. Results The gene expression level of CYP3A4 in HepG2 cells was significantly reduced by treatment with menthol for 1 day. The viability of HepG2 cells was not affected by treatment with menthol alone once a day for two consecutive days. The degree of reduction in cell viability by PAC or VIN in HepG2 cells was significantly increased by menthol treatment for 24 h prior to exposure to these anti-cancer drugs. Conclusions These results demonstrate that menthol enhanced the anti-tumor effects of PAC and VIN through the downregulation of CYP3A4 in HepG2 cells without exerting cytotoxic activity.

Review article: the physiological effects and safety of peppermint oil and its efficacy in irritable bowel syndrome and other functional disorders

BACKGROUND

Peppermint oil has been used for centuries as a treatment for gastrointestinal ailments. It has been shown to have several effects on gastrointestinal physiology relevant to clinical care and management.

AIM

To review the literature on peppermint oil regarding its metabolism, effects on gastrointestinal physiology, clinical use and efficacy, and safety.

METHODS

We performed a PubMed literature search using the following terms individually or in combination: peppermint, peppermint oil, pharmacokinetics, menthol, oesophagus, stomach, small intestine, gallbladder, colon, transit, dyspepsia, nausea, abdominal pain, and irritable bowel syndrome. Full manuscripts evaluating peppermint oil that were published through 15 July 2017 were reviewed. When evaluating therapeutic indications, only randomised clinical trials were included. References from selected manuscripts were used if relevant.

RESULTS

It appears that peppermint oil may have several mechanisms of action including: smooth muscle relaxation (via calcium channel blockade or direct enteric nervous system effects); visceral sensitivity modulation (via transient receptor potential cation channels); anti-microbial effects; anti-inflammatory activity; modulation of psychosocial distress. Peppermint oil has been found to affect oesophageal, gastric, small bowel, gall-bladder, and colonic physiology. It has been used to facilitate completion of colonoscopy and endoscopic retrograde cholangiopancreatography. Placebo controlled studies support its use in irritable bowel syndrome, functional dyspepsia, childhood functional abdominal pain, and post-operative nausea. Few adverse effects have been reported in peppermint oil trials.

CONCLUSION

Peppermint oil is a natural product which affects physiology throughout the gastrointestinal tract, has been used successfully for several clinical disorders, and appears to have a good safety profile.

Hepatotoxicity of Herbal Supplements Mediated by Modulation of Cytochrome P450

Herbal supplements are a significant source of drug-drug interactions (DDIs), herb-drug interactions, and hepatotoxicity. Cytochrome P450 (CYP450) enzymes metabolize a large number of FDA-approved pharmaceuticals and herbal supplements. This metabolism of pharmaceuticals and supplements can be augmented by concomitant use of either pharmaceuticals or supplements. The xenobiotic receptors constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) can respond to xenobiotics by increasing the expression of a large number of genes that are involved in the metabolism of xenobiotics, including CYP450s. Conversely, but not exclusively, many xenobiotics can inhibit the activity of CYP450s. Induction of the expression or inhibition of the activity of CYP450s can result in DDIs and toxicity. Currently, the United States (US) Food and Drug Administration does not require the investigation of the interactions of herbal supplements and CYP450s. This review provides a summary of herbal supplements that inhibit CYP450s, induce the expression of CYP450s, and/or whose toxicity is mediated by CYP450s.

Gastrointestinal Pharmacology

There is little evidence for most of the medications currently used to treat functional abdominal pain disorders (FAPDs) in children. Not only are there very few clinical trials, but also most have significant variability in the methods used and outcomes measured. Thus, the decision on the most appropriate pharmacological treatment is frequently based on adult studies or empirical data. In children, peppermint oil, trimebutine, and drotaverine have shown significant benefit compared with placebo, each of them in a single randomized clinical trial. A small study found that cyproheptadine was beneficial in the treatment of FAPDs in children. There are conflicting data regarding amitriptyline. While one small study found a significant benefit in quality of life compared with placebo, a large multicenter study found no benefit compared with placebo. The antidepressant, citalopram, failed to meet the primary outcomes in intention-to-treat and per-protocol analysis. Rifaximin has been shown to be efficacious in the treatment of adults with IBS. Those findings differ from studies in children where no benefit was found compared to placebo. To date, there are no placebo-controlled trials published on the use of linaclotide or lubiprostone in children. Alpha 2 delta ligands such as gabapentin and pregabalin are sometimes used in the care of this group of children, but no clinical trials are available in children with FAPDs. Similarly, novel drugs that have been approved for the care of irritable bowel with diarrhea in adults such as eluxadoline have yet to be studied in children.

CONCLUSIONS

Little data support the use of most medications commonly used to treat FAPDs in children. More randomized, placebo-controlled studies are needed to assess the efficacy of pharmacological interventions in the treatment of FAPDs in children.

Systemic exposure to menthol following administration of peppermint oil to paediatric patients

OBJECTIVE

Peppermint oil (PMO) has been used to treat abdominal ailments dating to ancient Egypt, Greece and Rome. Despite its increasing paediatric use, as in irritable bowel syndrome (IBS) treatment, the pharmacokinetics (PK) of menthol in children given PMO has not been explored.

DESIGN AND SETTING

Single-site, exploratory pilot study of menthol PK following a single 187 mg dose of PMO. Subjects with paediatric Rome II defined (IBS; n=6, male and female, 7-15 years of age) were enrolled. Blood samples were obtained before PMO administration and at 10 discrete time points over a 12 h postdose period. Menthol was quantitated from plasma using a validated gas chromatography mass spectrometry technique. Menthol PK parameters were determined using a standard non-compartmental approach.

RESULTS

Following a dose of PMO, a substantial lag time (range 1-4 h) was seen in all subjects for the appearance of menthol which in turn, produced a delayed time of peak (Tmax=5.3 ± 2.4 h) plasma concentration (Cmax=698.2 ± 245.4 ng/mL). Tmax and Tlag were significantly more variable than the two exposure parameters; Cmax, mean residence time and total area under the curve (AUC=4039.7 ± 583.8 ng/mL × h) which had a coefficient of variation of <20%.

CONCLUSIONS

Delayed appearance of menthol in plasma after oral PMO administration in children is likely a formulation-specific event which, in IBS, could increase intestinal residence time of the active ingredient. Our data also demonstrate the feasibility of using menthol PK in children with IBS to support definitive studies of PMO dose-effect relationships.

Change in pharmacokinetic behavior of intravenously administered midazolam due to increased CYP3A2 expression in rats treated with menthol

Menthol is used widely as a constituent of functional foods and chemical drugs. The present study investigated changes in the pharmacokinetic behavior of intravenously administered midazolam (MDZ), a probe for CYP3A, when rats were treated with menthol. The study also examined which isoforms of CYP3A1 and 3A2 were menthol-inducible and contributed to the altered disposition of midazolam. Menthol was administered intraperitoneally to rats once daily for 3 days at a dose of 10 mg/kg, while the control rats received vehicle alone. The pharmacokinetic examination of i.v. administered midazolam revealed that serum midazolam concentrations at each sampling point were lower in the menthol-treated rats than in the control rats. Regarding the pharmacokinetic parameters of the menthol-treated group, the area under the curve (AUC) was decreased significantly and, correspondingly, the elimination rate constant at terminal phase (ke) was increased significantly without significant changes in the volume of distribution at steady state (Vdss). The metabolic production of the 1'-hydroxylated and 4'-hydroxylated forms of MDZ by hepatic microsomes was significantly greater in the menthol-treated rats than in the control rats. The expression levels of mRNA and protein for hepatic CYP3A2 were more than 2.5-fold higher than the control levels when the rats were treated with menthol, whereas no changes were observed in the expression levels of CYP3A1. These results indicate that menthol enhanced the elimination clearance of midazolam by inducing hepatic CYP3A2 and that careful attention should be paid when menthol is ingested in combination with drugs that act as substrates for CYP3A.

Feasibility of detecting prostate cancer by ultraperformance liquid chromatography-mass spectrometry serum metabolomics

Prostate cancer (PCa) is the second leading cause of cancer-related mortality in men. The prevalent diagnosis method is based on the serum prostate-specific antigen (PSA) screening test, which suffers from low specificity, overdiagnosis, and overtreatment. In this work, untargeted metabolomic profiling of age-matched serum samples from prostate cancer patients and healthy individuals was performed using ultraperformance liquid chromatography coupled to high-resolution tandem mass spectrometry (UPLC-MS/MS) and machine learning methods. A metabolite-based in vitro diagnostic multivariate index assay (IVDMIA) was developed to predict the presence of PCa in serum samples with high classification sensitivity, specificity, and accuracy. A panel of 40 metabolic spectral features was found to be differential with 92.1% sensitivity, 94.3% specificity, and 93.0% accuracy. The performance of the IVDMIA was higher than the prevalent PSA test. Within the discriminant panel, 31 metabolites were identified by MS and MS/MS, with 10 further confirmed chromatographically by standards. Numerous discriminant metabolites were mapped in the steroid hormone biosynthesis pathway. The identification of fatty acids, amino acids, lysophospholipids, and bile acids provided further insights into the metabolic alterations associated with the disease. With additional work, the results presented here show great potential toward implementation in clinical settings.

Menthol: a simple monoterpene with remarkable biological properties

Menthol is a cyclic monoterpene alcohol which possesses well-known cooling characteristics and a residual minty smell of the oil remnants from which it was obtained. Because of these attributes it is one of the most important flavouring additives besides vanilla and citrus. Due to this reason it is used in a variety of consumer products ranging from confections such as chocolate and chewing gum to oral-care products such as toothpaste as well as in over-the-counter medicinal products for its cooling and biological effects. Its cooling effects are not exclusive to medicinal use. Approximately one quarter of the cigarettes on the market contain menthol and small amounts of menthol are even included in non-mentholated cigarettes. Natural menthol is isolated exclusively from Mentha canadensis, but can also be synthesised on industrial scale through various processes. Although menthol exists in eight stereoisomeric forms, (-)-menthol from the natural source and synthesised menthol with the same structure is the most preferred isomer. The demand for menthol is high and it was previously estimated that the worldwide use of menthol was 30-32,000 metric tonnes per annum. Menthol is not a predominant compound of the essential oils as it can only be found as a constituent of a limited number of aromatic plants. These plants are known to exhibit biological activity in vitro and in vivo such as antibacterial, antifungal, antipruritic, anticancer and analgesic effects, and are also an effective fumigant. In addition, menthol is one of the most effective terpenes used to enhance the dermal penetration of pharmaceuticals. This review summarises the chemical and biological properties of menthol and highlights its cooling effects and toxicity.

Inferring the metabolism of human orphan metabolites from their metabolic network context affirms human gluconokinase activity

Metabolic network reconstructions define metabolic information within a target organism and can therefore be used to address incomplete metabolic information. In the present study we used a computational approach to identify human metabolites whose metabolism is incomplete on the basis of their detection in humans but exclusion from the human metabolic network reconstruction RECON 1. Candidate solutions, composed of metabolic reactions capable of explaining the metabolism of these compounds, were then identified computationally from a global biochemical reaction database. Solutions were characterized with respect to how metabolites were incorporated into RECON 1 and their biological relevance. Through detailed case studies we show that biologically plausible non-intuitive hypotheses regarding the metabolism of these compounds can be proposed in a semi-automated manner, in an approach that is similar to de novo network reconstruction. We subsequently experimentally validated one of the proposed hypotheses and report that C9orf103, previously identified as a candidate tumour suppressor gene, encodes a functional human gluconokinase. The results of the present study demonstrate how semi-automatic gap filling can be used to refine and extend metabolic reconstructions, thereby increasing their biological scope. Furthermore, we illustrate how incomplete human metabolic knowledge can be coupled with gene annotation in order to prioritize and confirm gene functions.

Biotransformation of (-)-camphor by Salmonella typhimurium OY1002/2A6 expressing human CYP2A6 and NADPH-P450 reductase

In a previous in vitro study, (-)-camphor (1) was examined by incubation with human liver microsomes, and the oxidative metabolites thus formed were analyzed using gas chromatography-mass spectrometry. However, thus far, no large-scale biotransformation using recombinant human P450 has been performed. Here, the biotransformation of compound 1 has been investigated by using Salmonella typhimurium OY1002/2A6 expressing human CYP2A6 and human NADPH-P450 reductase as a biocatalyst. Compound 1 (400 mg) was converted to (1S,5S)-(-)-5-exo-hydroxycamphor (2) (30.4 mg) and (1S,7S)-(-)-8-hydroxycamphor (3) (2.4 mg) by S. typhimurium OY1002/2A6. This is the first report to show that large quantities of metabolites 2 and 3 can be produced by S. typhimurium OY1002/2A6 expressing human CYP2A6 and NADPH-P450 reductase.

Determination of cytochrome P450 enzymes involved in the metabolism of (-)-terpinen-4-ol by human liver microsomes

The in vitro metabolism of (-)-terpinen-4-ol was examined in human liver microsomes and recombinant enzymes. The biotransformation of (-)-terpinen-4-ol was investigated by gas chromatography-mass spectrometry. (-)-Terpinen-4-ol was found to be oxidized to (-)-(1S,2R,4R)-1,2-epoxy-p-menthan-4-ol, major metabolic product by human liver microsomal P450 enzymes. The formation of metabolites of (-)-terpinen-4-ol was determined by relative abundance of mass fragments and retention times on GC. CYP2A6 in human liver microsomes was a major enzyme involved in the oxidation of (-)-terpinen-4-ol by human liver microsomes, based on the following lines of evidence. First, of 11 recombinant human P450 enzymes tested, CYP2A6 had the highest activity for oxidation of (-)-terpinen-4-ol. Second, oxidation of (-)-terpinen-4-ol was inhibited by (+)-menthofuran. Finally, there was a good correlation between CYP2A6 maker activity and (-)-terpinen-4-ol oxidation activities in liver microsomes of 10 human samples. Kinetic analysis showed that the V(max)/K(m) values for (-)-(1S,2R,4R)-1,2-epoxy-p-menthan-4-ol catalysed by liver microsomes of human sample HH-18 was 2.49 μL/min/nmol. Human recombinant CYP2A6 catalysed (-)-(1S,2R,4R)-1,2-epoxy-p-menthan-4-ol with V(max) values of 13.9 nmol/min/nmol P450 and apparent K(m) values of 91 μM.

Metabolism of (+)-terpinen-4-ol by cytochrome P450 enzymes in human liver microsomes

We examined the in vitro metabolism of (+)-terpinen-4-ol by human liver microsomes and recombinant enzymes. The biotransformation of (+)-terpinen-4-ol was investigated by gas chromatography-mass spectrometry (GC-MS). (+)-Terpinen-4-ol was found to be oxidized to (+)-(1R,2S,4S)-1,2-epoxy-p-menthan-4-ol, (+)-(1S,2R,4S)-1,2-epoxy-p-menthan-4-ol, and (4S)-p-menth-1-en-4,8-diol by human liver microsomal P450 enzymes. The identities of (+)-terpinen-4-ol metabolites were determined through the relative abundance of mass fragments and retention times on GC-MS. Of 11 recombinant human P450 enzymes tested, CYP1A2, CYP2A6, and CYP3A4 were found to catalyze the oxidation of (+)-terpinen-4-ol. Based on several lines of evidence, CYP2A6 and CYP3A4 were determined to be major enzymes involved in the oxidation of (+)-terpinen-4-ol by human liver microsomes. First, of the 11 recombinant human P450 enzymes tested, CYP1A2, CYP2A6 and CYP3A4 catalyzed oxidation of (+)-terpinen-4-ol. Second, oxidation of (+)-terpinen-4-ol was inhibited by (+)-menthofuran and ketoconazole, inhibitors known to be specific for these enzymes. Finally, there was a good correlation between CYP2A6 and CYP3A4 activities and (+)-terpinen-4-ol oxidation activities in the 10 human liver microsomes.