The metabolism of 14C-labelled trimethylamine and its N-oxide in man

Involvement of Organic Cation Transporters in the Kinetics of Trimethylamine N-oxide

Recent studies suggest that trimethylamine N-oxide (TMAO) is associated with the development of chronic kidney disease and heart failure. In this study, we investigated the importance of organic cation transporters (OCTs) in the clearance and tissue distribution of TMAO. The low-affinity and high-capacity transport of TMAO by mouse and human OCT1 and OCT2 was observed. Uptake and efflux of TMAO by the mouse hepatocytes as well as TMAO uptake into mouse kidney slices were significantly decreased by the addition of tetraethylammonium or Oct1/2 double knockout (dKO). Plasma concentrations of endogenous TMAO and TMAO-d9 given by intravenous infusion was 2-fold higher in Oct1/2 dKO than in wild-type mice due to significant decrease in its renal clearance. These results indicate that OCTs have a crucial role in the kinetics of TMAO in mice. In human, however, the OCT2-mediated tubular secretion in the urinary excretion of TMAO was insignificant because the renal clearance of TMAO was similar to that of creatinine in both young and elderly subjects, suggesting the species difference in the urinary excretion mechanisms of TMAO between mouse and human.

Levocarnitine Injections Decrease the Need for Erythropoiesis-Stimulating Agents in Hemodialysis Patients with Renal Anemia

AIMS

The aim of this study was to evaluate the efficacy of levocarnitine injection for renal anemia in hemodialysis patients.

METHODS

In this randomized controlled clinical trial, we randomly assigned patients on maintenance hemodialysis at our hospital to receive levocarnitine injections (n = 30) or no injection (n = 30) and monitored the patients during 12 months of treatment. In the treatment group, patients received an injection of levocarnitine 1,000 mg 3 times weekly after hemodialysis sessions. All patients received recombinant human erythropoietin as an erythropoiesis-stimulating agent (ESA). Response to ESA therapy was determined by calculating the erythropoietin responsiveness index (ERI; ESA dose·kg^-1·g^-1· dL^-1·week^-1).

RESULTS

(1) The target levels of hemoglobin and hematocrit were maintained during the study period in both the levocarnitine group and the control group. (2) The dose of ESAs required to maintain these levels decreased gradually in the levocarnitine group and was significantly lower at 6 and 12 months than at study initiation. Furthermore, the dose of ESAs was significantly lower than that in the control group at 12 months. (3) The ERI showed a significant decrease at 6 and 12 months in the levocarnitine group, with a significant difference between the 2 groups at 12 months.

CONCLUSION

Our results suggest that levocarnitine administration can reduce the dose of ESAs required in patients with renal anemia on hemodialysis and improve the response to ESA therapy.

An overview of renal metabolomics

The high-throughput, high-resolution phenotyping enabled by metabolomics has been applied increasingly to a variety of questions in nephrology research. This article provides an overview of current metabolomics methodologies and nomenclature, citing specific considerations in sample preparation, metabolite measurement, and data analysis that investigators should understand when examining the literature or designing a study. Furthermore, we review several notable findings that have emerged in the literature that both highlight some of the limitations of current profiling approaches, as well as outline specific strengths unique to metabolomics. More specifically, we review data on the following: (i) tryptophan metabolites and chronic kidney disease onset, illustrating the interpretation of metabolite data in the context of established biochemical pathways; (ii) trimethylamine-N-oxide and cardiovascular disease in chronic kidney disease, illustrating the integration of exogenous and endogenous inputs to the blood metabolome; and (iii) renal mitochondrial function in diabetic kidney disease and acute kidney injury, illustrating the potential for rapid translation of metabolite data for diagnostic or therapeutic aims. Finally, we review future directions, including the need to better characterize interperson and intraperson variation in the metabolome, pool existing data sets to identify the most robust signals, and capitalize on the discovery potential of emerging nontargeted methods.

Identification and Characterization of Trimethylamine-N-oxide Uptake and Efflux Transporters

Trimethylamine-N-oxide (TMAO) is a recently identified predictor of cardiovascular and chronic kidney disease. TMAO is primarily generated through gut-microbiome mediated conversion of dietary choline and carnitine to TMA, which is converted to TMAO by hepatic flavin monooxygenase 3 (FMO3) and subsequently undergoes renal elimination. We investigated the role of uptake and efflux drug transporters in TMAO disposition in vitro and in vivo. After screening a large array of uptake transporters, we show organic cation transporter 2 (OCT2) is the key transporter for TMAO cellular uptake. In Oct1/2 knockout mice, we observed increased plasma TMAO levels with reduced renal retention, suggesting the importance of Oct2 in facilitating the uptake of TMAO into renal tubular cells in vivo. Multiple transporters of the ATP-binding cassette (ABC) family, including ABCG2 (BCRP) and ABCB1 (MDR1), were capable of TMAO efflux. In human subjects, clinical, dietary, and pharmacogenetic covariates were evaluated for contribution to TMAO levels in a cohort of dyslipidemic patients (n = 405). Interestingly, genetic variation in ABCG2, but not other transporters, appeared to play a role in modulating TMAO exposure.

Exploring the Microbiome in Heart Failure

Recent years have brought interesting insights into the human gut microbiota and have highlighted its increasingly recognized impact on cardiovascular (CV) diseases, including heart failure (HF). Changes in composition of gut microbiota, called dysbiosis, can trigger systemic inflammation, which is known to be involved in the pathophysiology of HF. Trimethylamine N-oxide (TMAO), which is derived from gut microbiota metabolites of specific dietary nutrients, has emerged as a key contributor to cardiovascular disease pathogenesis. Elevated TMAO levels have been reported to be associated with poor outcomes in patients with both HF and chronic kidney disease (CKD). Dysbiosis of gut microbiota can contribute to higher levels of TMAO and the generation of uremic toxins, progressing to both HF and CKD. Therefore, this bidirectional relationship between HF and CKD through gut microbiota may be a novel therapeutic target for the cardiorenal syndrome. However, the mechanisms by which gut microbiota could influence the development of heart failure are still unknown, and there are still some questions regarding the causative effects of TMAO and the underlying mechanistic link that explains how TMAO might directly or indirectly promote CV diseases including HF. Further studies are warranted to clarify the function of TMAO on the pathophysiology of cardiorenal syndrome and the handling of TMAO levels by the kidneys.

Trimethylamine N-Oxide: The Good, the Bad and the Unknown

Trimethylamine N-oxide (TMAO) is a small colorless amine oxide generated from choline, betaine, and carnitine by gut microbial metabolism. It accumulates in the tissue of marine animals in high concentrations and protects against the protein-destabilizing effects of urea. Plasma level of TMAO is determined by a number of factors including diet, gut microbial flora and liver flavin monooxygenase activity. In humans, a positive correlation between elevated plasma levels of TMAO and an increased risk for major adverse cardiovascular events and death is reported. The atherogenic effect of TMAO is attributed to alterations in cholesterol and bile acid metabolism, activation of inflammatory pathways and promotion foam cell formation. TMAO levels increase with decreasing levels of kidney function and is associated with mortality in patients with chronic kidney disease. A number of therapeutic strategies are being explored to reduce TMAO levels, including use of oral broad spectrum antibiotics, promoting the growth of bacteria that utilize TMAO as substrate and the development of target-specific molecules with varying level of success. Despite the accumulating evidence, it is questioned whether TMAO is the mediator of a bystander in the disease process. Thus, it is important to undertake studies examining the cellular signaling in physiology and pathological states in order to establish the role of TMAO in health and disease in humans.

Associations among serum trimethylamine-N-oxide (TMAO) levels, kidney function and infarcted coronary artery number in patients undergoing cardiovascular surgery: a cross-sectional study

BACKGROUND

Trimethylamine-N-oxide (TMAO) is a metabolite of phosphatidylcholine generated by gut microbiota and liver enzymes, and has recently been recognized as contributing to atherosclerosis. Elevated serum TMAO levels have been shown to increase the risk of cardiovascular disease (sudden death, myocardial infarction, or stroke) in patients undergoing elective coronary angiography. We aimed to clarify whether TMAO levels are associated with the number of infarcted coronary arteries as a measure of the severity of atherosclerosis, with adjustment using a priori-defined covariates such as kidney function.

METHODS

By conducting a cross-sectional study of 227 patients who underwent cardiovascular surgery for coronary artery disease, valvular heart disease, or aortic disease, the association between serum TMAO levels as measured by HPLC-APCI-MS/MS and the number of infarcted coronary arteries was evaluated using ordered logistic regression models with adjustment of 10 covariates, including chronic kidney disease (CKD) stage. Unadjusted and adjusted odds ratios (ORs) and 95 % confidence intervals (95 % CIs) were determined.

RESULTS

Significantly higher TMAO levels were observed in advanced-stage CKD (p ≤ 0.001). In fully adjusted models with the 10 covariates, a significantly increased number of infarcted coronary arteries was identified in the highest quartile and quintile of TMAO compared to the lowest quartile (OR 11.9; 95 % CI 3.88-36.7, p ≤ 0.001) and quintile (OR 14.1; 95 % CI 3.88-51.2; p ≤ 0.001), respectively, independent of dyslipidemia.

CONCLUSIONS

Higher serum TMAO levels may be associated with advanced CKD stages and with an increased number of infarcted coronary arteries in patients who undergo cardiovascular surgery.

Trimethylamine and Trimethylamine N-Oxide, a Flavin-Containing Monooxygenase 3 (FMO3)-Mediated Host-Microbiome Metabolic Axis Implicated in Health and Disease

Flavin-containing monooxygenase 3 (FMO3) is known primarily as an enzyme involved in the metabolism of therapeutic drugs. On a daily basis, however, we are exposed to one of the most abundant substrates of the enzyme trimethylamine (TMA), which is released from various dietary components by the action of gut bacteria. FMO3 converts the odorous TMA to nonodorous TMA N-oxide (TMAO), which is excreted in urine. Impaired FMO3 activity gives rise to the inherited disorder primary trimethylaminuria (TMAU). Affected individuals cannot produce TMAO and, consequently, excrete large amounts of TMA. A dysbiosis in gut bacteria can give rise to secondary TMAU. Recently, there has been much interest in FMO3 and its catalytic product, TMAO, because TMAO has been implicated in various conditions affecting health, including cardiovascular disease, reverse cholesterol transport, and glucose and lipid homeostasis. In this review, we consider the dietary components that can give rise to TMA, the gut bacteria involved in the production of TMA from dietary precursors, the metabolic reactions by which bacteria produce and use TMA, and the enzymes that catalyze the reactions. Also included is information on bacteria that produce TMA in the oral cavity and vagina, two key microbiome niches that can influence health. Finally, we discuss the importance of the TMA/TMAO microbiome-host axis in health and disease, considering factors that affect bacterial production and host metabolism of TMA, the involvement of TMAO and FMO3 in disease, and the implications of the host-microbiome axis for management of TMAU.

Serum Trimethylamine-N-Oxide is Elevated in CKD and Correlates with Coronary Atherosclerosis Burden

Trimethlyamine-N-oxide (TMAO) was recently identified as a promoter of atherosclerosis. Patients with CKD exhibit accelerated development of atherosclerosis; however, no studies have explored the relationship between TMAO and atherosclerosis formation in this group. This study measured serum concentrations and urinary excretion of TMAO in a CKD cohort (n=104), identified the effect of renal transplant on serum TMAO concentration in a subset of these patients (n=6), and explored the cross-sectional relationship between serum TMAO and coronary atherosclerosis burden in a separate CKD cohort (n=220) undergoing coronary angiography. Additional exploratory analyses examined the relationship between baseline serum TMAO and long-term survival after coronary angiography. Serum TMAO concentrations demonstrated a strong inverse association with eGFR (r(2)=0.31, P<0.001). TMAO concentrations were markedly higher in patients receiving dialysis (median [interquartile range], 94.4 μM [54.8-133.0 μM] for dialysis-dependent patients versus 3.3 μM [3.1-6.0 μM] for healthy controls; P<0.001); whereas renal transplantation resulted in substantial reductions in TMAO concentrations (median [min-max] 71.2 μM [29.2-189.7 μM] pretransplant versus 11.4 μM [8.9-20.2 μM] post-transplant; P=0.03). TMAO concentration was an independent predictor for coronary atherosclerosis burden (P=0.02) and predicted long-term mortality independent of traditional cardiac risk factors (hazard ratio, 1.26 per 10 μM increment in TMAO concentration; 95% confidence interval, 1.13 to 1.40; P<0.001). In conclusion, serum TMAO concentrations substantially increase with decrements in kidney function, and this effect is reversed by renal transplantation. Increased TMAO concentrations correlate with coronary atherosclerosis burden and may associate with long-term mortality in patients with CKD undergoing coronary angiography.

Gut microbiota-dependent trimethylamine N-oxide (TMAO) pathway contributes to both development of renal insufficiency and mortality risk in chronic kidney disease

RATIONALE

Trimethylamine-N-oxide (TMAO), a gut microbial-dependent metabolite of dietary choline, phosphatidylcholine (lecithin), and l-carnitine, is elevated in chronic kidney diseases (CKD) and associated with coronary artery disease pathogenesis.

OBJECTIVE

To both investigate the clinical prognostic value of TMAO in subjects with versus without CKD, and test the hypothesis that TMAO plays a direct contributory role in the development and progression of renal dysfunction.

METHODS AND RESULTS

We first examined the relationship between fasting plasma TMAO and all-cause mortality over 5-year follow-up in 521 stable subjects with CKD (estimated glomerular filtration rate, <60 mL/min per 1.73 m(2)). Median TMAO level among CKD subjects was 7.9 μmol/L (interquartile range, 5.2-12.4 μmol/L), which was markedly higher (P<0.001) than in non-CKD subjects (n=3166). Within CKD subjects, higher (fourth versus first quartile) plasma TMAO level was associated with a 2.8-fold increased mortality risk. After adjustments for traditional risk factors, high-sensitivity C-reactive protein, estimated glomerular filtration rate, elevated TMAO levels remained predictive of 5-year mortality risk (hazard ratio, 1.93; 95% confidence interval, 1.13-3.29; P<0.05). TMAO provided significant incremental prognostic value (net reclassification index, 17.26%; P<0.001 and differences in area under receiver operator characteristic curve, 63.26% versus 65.95%; P=0.036). Among non-CKD subjects, elevated TMAO levels portend poorer prognosis within cohorts of high and low cystatin C. In animal models, elevated dietary choline or TMAO directly led to progressive renal tubulointerstitial fibrosis and dysfunction.

CONCLUSIONS

Plasma TMAO levels are both elevated in patients with CKD and portend poorer long-term survival. Chronic dietary exposures that increase TMAO directly contributes to progressive renal fibrosis and dysfunction in animal models.

Breath ammonia and trimethylamine allow real-time monitoring of haemodialysis efficacy

Non-invasive monitoring of breath ammonia and trimethylamine using Selected-ion-flow-tube mass spectroscopy (SIFT-MS) could provide a real-time alternative to current invasive techniques. Breath ammonia and trimethylamine were monitored by SIFT-MS before, during and after haemodialysis in 20 patients. In 15 patients (41 sessions), breath was collected hourly into Tedlar bags and analysed immediately (group A). During multiple dialyses over 8 days, five patients breathed directly into the SIFT-MS analyser every 30 min (group B). Pre- and post-dialysis direct breath concentrations were compared with urea reduction, Kt/V and creatinine concentrations. Dialysis decreased breath ammonia, but a transient increase occurred mid treatment in some patients. Trimethylamine decreased more rapidly than reported previously. Pre-dialysis breath ammonia correlated with pre-dialysis urea in group B (r(2) = 0.71) and with change in urea (group A, r(2) = 0.24; group B, r(2) = 0.74). In group B, ammonia correlated with change in creatinine (r(2) = 0.35), weight (r(2) = 0.52) and Kt/V (r(2) = 0.30). The ammonia reduction ratio correlated with the urea reduction ratio (URR) (r(2) = 0.42) and Kt/V (r(2) = 0.38). Pre-dialysis trimethylamine correlated with Kt/V (r(2) = 0.21), and the trimethylamine reduction ratio with URR (r(2) = 0.49) and Kt/V (r(2) = 0.36). Real-time breath analysis revealed previously unmeasurable differences in clearance kinetics of ammonia and trimethylamine. Breath ammonia is potentially useful in assessment of dialysis efficacy.

Short-chain aliphatic amines in human urine: a mathematical examination of metabolic interrelationships

The relationships between several small molecular weight aliphatic amines (methylamine, dimethylamine, trimethylamine, and ethylamine) and an associated N-oxide (trimethylamine N-oxide) quantified in human urine collected from 203 healthy volunteers have been assessed mathematically. Principal component analysis highlighted a female subgroup with raised trimethylamine levels and the possibility of hormonal influence on the N-oxidation of trimethylamine has been proposed. A second subgroup of men, who ate a large meal of fish before the study, displayed raised levels of all compounds except ethylamine. In all cases, ethylamine was least significantly correlated with the other urinary components and appeared metabolically unrelated.

Accumulation of trimethylamine and trimethylamine-N-oxide in end-stage renal disease patients undergoing haemodialysis

BACKGROUND

Trimethylamine (TMA) is a short-chain tertiary aliphatic amine that is derived from the diet either directly from the consumption of foods high in TMA or by the intake of food high in precursors to TMA, such as trimethylamine-N-oxide (TMNO), choline and L-carnitine. The clinical significance of TMA may be related to its potential to contribute to neurological toxicity and 'uraemic breath' in patients with end-stage renal disease (ESRD).

METHODS

Concentrations of TMA and TMNO in plasma from 10 healthy adults (not on haemodialysis) and 10 adults with ESRD undergoing haemodialysis (pre- and post-dialysis) were determined by gas chromatography-mass spectrometry.

RESULTS

The concentrations of TMA and TMNO in pre-dialysis plasma (1.39+/-0.483 and 99.9+/-31.9 microM, respectively) were significantly (P<0.05) higher than the corresponding levels in healthy subjects (0.418+/-0.124 and 37.8+/-20.4 microM, respectively). However, there were no significant differences between post-dialysis and healthy subject plasma concentrations. In the ESRD patients, there was a significant (P<0.05) reduction in plasma TMA (from 1.39+/-0.483 to 0.484+/-0.164 microM) and TMNO (from 99.9+/-31.9 to 41.3+/-18.8 microM) during a single haemodialysis session.

CONCLUSIONS

TMA and TMNO accumulate between haemodialysis sessions in ESRD patients, but are efficiently removed during a single haemodialysis session.

The passage of trimethylamine across rat and human skin

Trimethylamine is a volatile low molecular weight tertiary aliphatic amine that has known toxicity and the potential for human exposure from industrial and environmental sources is considerable. It is generally believed that absorption across the skin is an unimportant route of entry but there is little, if any, supporting evidence for this assumption. Passage across rat and human skin has been investigated employing excised skin circles in an in vitro diffusion cell apparatus. Trimethylamine was found to penetrate readily when applied to the epidermal surface of skin at three different dose levels (0.1, 1.0 and 10 mg per skin membrane 0.32 cm2). The apparent dermal flux was calculated as 3.40 +/- 1.60, 58.3 +/- 30.6 and 265.0 +/- 155.0 microg/cm2/h for rat and 0.98 +/- 0.75, 9.21 +/- 3.06 and 92.7 +/- 31.9 microg/cm2/h for human at the three dose levels, respectively. Both rat and human skin was able to act as a reservoir, with the trimethylamine not remaining in the stratum corneum but passing through. When presented to the underneath of rat and human skin circles, both [U-14C]-trimethylamine and [U-14C]-trimethylamine N-oxide were able to pass from the dermis to the epidermis. Small but detectable amounts of trimethylamine were oxidised to its N-oxide during passage through the skin.

Trimethylamine and odorous sweat

Analysis of sweat collected from patients experiencing ichthyohidriosis, and from volunteers in whom this odour phenomenon had been artificially induced, showed that trimethylamine was the major causative factor.

A nonsense mutation in the FMO3 gene underlies fishy off-flavor in cow's milk

Fish-odor syndrome or Trimethylaminuria (OMIM #602079) in humans is an inborn error of metabolism associated with a characteristic fishy body odor due to elevated levels of trimethylamine (TMA) in body fluids. It is caused by loss-of-function mutations in FMO3 encoding flavin-containing mono-oxygenase 3. A fishy off-flavor is occasionally observed in cow's milk and it has been established recently that this phenotype is due to elevated TMA levels. Here, we report that fishy off-flavor in cow's milk is caused by a nonsense mutation (R238X) in the bovine FMO3 ortholog. RT-PCR analysis indicated that the mutant transcript is present in a very low amount. The mutation was found to be surprisingly common (q = 0.155) in one breed of cattle.

Compound heterozygosity for missense mutations in the flavin-containing monooxygenase 3 (FM03) gene in patients with fish-odour syndrome

Fish-odour syndrome is a highly unpleasant disorder of hepatic trimethylamine (TMA) metabolism characterized by a body odour reminiscent of rotting fish, due to excessive excretion of the malodorous free amine. Although fish-odour syndrome may exhibit as sequelae with other conditions (e.g. liver dysfunction), many patients exhibit an inherited, more persistent form of the disease. Ordinarily, dietary-derived TMA is oxidized to the nonodorous N-oxide by hepatic flavin-containing monooxygenase 3 (FMO3). Our previous demonstration that a mutation, P153L (C to T), in the FMO3 gene segregated with the disorder and inactivated the enzyme confirmed that defects in FMO3 underlie the inherited form of fish-odour syndrome. We have investigated the genetic basis of the disorder in two further affected pedigrees and report that the three propositi are all compound heterozygotes for causative mutations of FMO3. Two of these individuals possess the P153L (C to T) mutation and a novel mutation, N61S (A to G). The third is heterozygous for novel, M4341 (G to A), and previously reported, R492W (C to T), mutations. Functional characterization of the S61, 1434 and W492 variants, via baculovirus-mediated expression in insect cells, confirmed that all three mutations either abolished, or severely attenuated, the capacity of the enzyme to catalyse TMA N-oxidation. Although 1434 and W492 were also incapable of catalysing the S-oxidation of methimazole, S61 was fully active with this sulphur-containing substrate. Since an asparagine is conserved at the equivalent position to N61 of FMO3 in mammalian, yeast and Caenorhabditis elegans FMOs, the characterization of the naturally occurring N61S (A to G) mutation may have identified this asparagine as playing a critical role specifically in FMO-catalysed N-oxidation.

A novel mutation in the flavin-containing monooxygenase 3 gene, FM03, that causes fish-odour syndrome: activity of the mutant enzyme assessed by proton NMR spectroscopy

We have previously shown that primary trimethylaminuria, or fish-odour syndrome, is caused by an inherited defect in the flavin-containing monooxygenase 3 (FMO3) catalysed N-oxidation of the dietary-derived malodorous amine, trimethylamine (TMA). We now report a novel causative mutation for the disorder identified in a young girl diagnosed by proton nuclear magnetic resonance (NMR) spectroscopy of her urine. Sequence analysis of genomic DNA amplified from the patient revealed that she was homozygous for a T to C missense mutation in exon 3 of the FMO3 gene. The mutation changes an ATG triplet, encoding methionine, at codon 82 to an ACG triplet, encoding threonine. A polymerase chain reaction/restriction enzyme-based assay was devised to genotype individuals for the FMO3Thr82 allele. Wild-type and mutant FMO3, heterologously expressed in a baculovirus-insect cell system, were assayed by ultraviolet spectrophotometry and NMR spectroscopy for their ability to catalyse the N-oxidation of TMA. The latter technique has the advantage of enabling the simultaneous, direct and semi-continuous measurement of both of the products, TMA N-oxide and NADP, and of one of the reactants, NADPH. Results obtained from both techniques demonstrate that the Met82Thr mutation abolishes the catalytic activity of the enzyme and thus represents the genetic basis of the disorder in this individual. The combination of NMR spectroscopy with gene sequence and expression technology provides a powerful means of determining genotype-phenotype relationships in trimethylaminuria.

The suncus (Suncus murinus) shows poor metabolic phenotype for trimethylamine N-oxygenation

In vitro and in vivo N-oxygenation of trimethylamine (TMA) in the suncus (Suncus murinus) was investigated. The N-oxygenation of TMA has been thought to be catalyzed by flavin-containing monooxygenase (FMO). In a previous study, we found that the levels of mRNAs for FMOs were extremely low in the suncus. Thus, we intended to evaluate the capacity of the suncus to N-oxygenate TMA compared to the rat. Eadie-Hofstee plots of the TMA N-oxygenation by suncus liver microsomes showed a biphasic pattern, suggesting that more than two enzymes were involved in this reaction. The low K(m) component in the suncus showed a twofold higher K(m) (55 vs. 31 microM) and a fourfold lower V(max) (0.61 vs 2.5 nmol/min/mg protein) values than those obtained using rat liver microsomes, resulting in a sevenfold lower V(max)/K(m) (11 vs 82 microl/min/mg protein) value. After an intraperitoneal administration of TMA (10 mg/kg body wt), the suncus excreted 39.6% of the dose in 24-h urine as TMA, whereas the rats excreted 6.3%. Metabolic ratio in the TMA N-oxygenation was 1.42 and 0.11 in the suncus and the rat, respectively. These results indicate that the suncus can be an animal model for a poor metabolizer phenotype in TMA metabolism.

Sensitive assay of trimethylamine N-oxide in liver microsomes by headspace gas chromatography with flame thermionic detection

To compare the trimethylamine N-oxygenase activity of liver microsomes from house musk shrew (Suncus murinus) and rat, a sensitive method for the quantitation of trimethylamine (TMA) N-oxide was developed using gas chromatography with flame thermionic detection. The limit of quantification was 0.5 microM and the calibration curve was linear at least up to 5 microM in incubations containing liver microsomal preparations from Suncus. The intra-day RSD values ranged from 10.4 to 12.8 at 0.5 microM and from 3.5 to 6.7 at 5 microM. The inter-day RSD values were 11.6 and 6.5 at 0.5 and 5 microM, respectively. This method provides a sensitive assay for TMA N-oxygenase activity in liver microsomes. Using this method we found that Suncus was capable of N-oxidizing trimethylamine at a very slow rate.

In vitro and in vivo inhibition of human flavin-containing monooxygenase form 3 (FMO3) in the presence of dietary indoles

The effect of consumption of glucosinolate-containing Brussels sprouts on flavin-containing monooxygenase functional activity in humans was investigated in 10 healthy, male, non-smoking volunteers. After a 3-week run-in period, 5 volunteers continued on a glucosinolate-free diet for 3 weeks (control group), and 5 others consumed 300 g of cooked Brussels sprouts per day (sprouts group). Human flavin-containing monooxygenase activity was measured by determining the levels of urinary trimethylamine and trimethylamine N-oxide. In the control group similar trimethylamine to trimethylamine N-oxide ratios were observed, while in the sprouts group the trimethylamine to trimethylamine N-oxide ratios were increased 2.6- to 3.2-fold, and thus flavin-containing monooxygenase functional activity was decreased significantly. To investigate the molecular basis for the in vivo inhibition of functional human flavin-containing monooxygenase activity, in vitro studies were carried out examining the effect of acid condensation products of indole-3-carbinol, anticipated to be formed after transit of Brussels sprouts through the gastrointestinal system, on the prominent cDNA-expressed human flavin-containing monooxygenase form 3 enzymes. Two indole-containing materials were observed to be potent inhibitors of human flavin-containing monooxygenases, having Ki values in the low micromolar range. The results suggested that acid condensation products expected to be formed upon transit of Brussels sprouts materials through the gastrointestinal system were potent competitive inhibitors of human flavin-containing monooxygenase form 3 enzymes. The findings indicate that daily intake of Brussels sprouts may lead to a decrease in human flavin-containing monooxygenase activity, and this may have consequences for metabolism of other xenobiotics or dietary constituents.

Dietary precursors of trimethylamine in man: a pilot study

An increased urinary excretion of trimethylamine and its N-oxide were observed in man following the oral intake (15 mmol) of choline (63% dose as trimethylamine and its N-oxide), D,L-carnitine (31% dose) and trimethylamine N-oxide (78% dose). Similar ingestion of betaine, creatinine or lecithin failed to elicit any significant increases. Of 46 different foods investigated, only fish and other sea-products gave rise to significant increases in urinary trimethylamine and N-oxide. Ingestion of fruits, vegetables, cereal and dairy produce, and meats had no measurable effects. Reasons for the apparent lack of trimethylamine provision by foods previously thought to be precursors are given and the role of gut microflora highlighted.

Isoform specificity of trimethylamine N-oxygenation by human flavin-containing monooxygenase (FMO) and P450 enzymes: selective catalysis by FMO3

In the present study, we expressed human flavin-containing monooxygenase 1 (FMO1), FMO3, FMO4t (truncated), and FMO5 in the baculovirus expression vector system at levels of 0.6 to 2.4 nmol FMO/mg of membrane protein. These four isoforms, as well as purified rabbit FMO2, and eleven heterologously expressed human P450 isoforms were examined for their capacity to metabolize trimethylamine (TMA) to its N-oxide (TMAO), using a new, specific HPLC method with radiochemical detection. Human FMO3 was by far the most active isoform, exhibiting a turnover number of 30 nmol TMAO/nmol FMO3/min at pH 7.4 and 0.5 mM TMA. None of the other monooxygenases formed TMAO at rates greater than 1 nmol/nmol FMO/min under these conditions. Human fetal liver, adult liver, kidney and intestine microsomes were screened for TMA oxidation, and only human adult liver microsomes provided substantial TMAO-formation (range 2.9 to 9.1 nmol TMAO/mg protein/min, N = 5). Kinetic studies of TMAO formation by recombinant human FMO3, employing three different analytical methods, resulted in a Km of 28 +/- 1 microM and a Vmax of 36.3 +/- 5.7 nmol TMAO/nmol FMO3/min. The Km determined in human liver microsomes ranged from 13.0 to 54.8 microM. Therefore, at physiological pH, human FMO3 is a very specific and efficient TMA N-oxygenase, and is likely responsible for the metabolic clearance of TMA in vivo in humans. In addition, this specificity provides a good in vitro probe for the determination of FMO3-mediated activity in human tissues, by analyzing TMAO formation at pH 7.4 with TMA concentrations not higher than 0.5 mM.

L-Carnitine moiety assay: an up-to-date reappraisal covering the commonest methods for various applications

L-Carnitine and its esters are typical endogenous substances. Their homeostatic equilibria are effectively controlled by various mechanisms which include rate-limiting enteral absorption, a multicomponent endogenous pool which is regulated according to a mammillary metabolism, an asymmetric body distribution and a saturable tubular reabsorption process leading to renal thresholds. In formal pharmacokinetic and metabolic investigations, the whole L-carnitine pool should be investigated, owing to the rapid interchange process between the various components of the pool. Free L-carnitine, as well as its acyl esters, must therefore be considered from an analytical viewpoint. L-Carnitine, acetyl-L-carnitine and total L-carnitine (the latter as an expression of the whole pool) can easily be assayed by enzyme or radioenzyme methods. Propionyl-L-carnitine and other esters containing fatty acids with more than three carbon atoms can be assayed using various HPLC approaches. Tandem mass spectrometry is another excellent approach to the assay of carnitine and its short-chain, medium-chain and long-chain esters. As L-carnitine contains a chiral carbon atom, the enantioselectivity of the assays is also considered in this review. Metabolites produced by enteral bacteria, namely tri-, di- and mono-methylamine, gamma-butyrobetaine, along with other systemic metabolites, namely trimethylamine N-oxide and N-nitroso dimethylamine, are very important in quantitative and toxicokinetic terms and require specific assay methods. This review covers the commonest methods of assaying carnitine and its esters, their impurities and pre-systemic and systemic metabolites and gives analytical details and information on their applications in pharmaceutics, biochemistry, pharmacokinetics and toxicokinetics.

1H NMR spectra of normal urines: reference ranges of the major metabolites

Serial urine samples from 50 normal subjects were studied by 1H NMR spectroscopy operating at 300 MHz. Analyses of the spectra have shown the presence of the following metabolites in 100% of the normal subjects: Creatinine, lactate, alanine, citrate, dimethylamine, trimethylamine-N-oxide, glycine and hippurate. Other analytes, such as creatine, valine, betaine, leucine and isoleucine, were sometimes found. All metabolites were quantified on the basis of peak heights and were expressed as mmol/mol of creatinine. The study of metabolic profiles in serial samples allowed us to evaluate intra-individual variability and physiological changes due to feeding. The aim of our report is to define standard conditions for this analytical technique and to calculate confidence intervals for the major metabolites in normal urine samples, such as preliminary and mandatory stages for clinical diagnostic 1H NMR utilization.

Discontinuous distribution of N-oxidation of dietary-derived trimethylamine in a British population

1. Whilst the majority of individuals within a British white population are able to convert > 90% of their dietary-derived trimethylamine to its N-oxide, outliers exist who show varying degrees of decreased metabolism. Such individuals, excrete unoxidized trimethylamine in their urine and, if N-oxidation is sufficiently low, may experience malodour problems (Fish-Odour syndrome). 2. Such observations have now been extended to a much larger group (n = 421; 221 males) of British white volunteers recruited from staff and students of Imperial College Medical School at St. Mary's, London. Each subject collected a 0-24-h urine sample, which was subsequently analysed for total trimethylamine and trimethylamine N-oxide content. 3. Sixteen subjects (3.8% population; seven male, nine female) excreted < 90% of their total trimethylamine output as N-oxide. All six subjects who excreted < 80% as N-oxide (indicative of potential heterozygous status for deficient N-oxidation-fish odour syndrome) were female.

Effect of dietary intake of trimethylamine on human metabolism of the industrial catalyst dimethylethylamine

OBJECTIVES

The aim was to study the effect of trimethylamine (TMA) on the metabolism of the industrial catalyst dimethylethylamine (DMEA) to ascertain whether biological monitoring of industrial exposure to DMEA is compromised and excretion of the malodorous DMEA in sweat and urine is increased by dietary intake of TMA.

METHODS

DMEA (0/25 mg) and TMA (0/300/600 mg) were given simultaneously once weekly for six weeks to five healthy volunteers. Plasma was collected before and one hour after the doses, and urine 0-2, 2-4, 4-6, 6-8, and 8-24 hours after the doses. Specimens were analysed by gas chromatography with a nitrogen sensitive detector.

RESULTS

Both amines were readily absorbed from the gastrointestinal tract and excreted in urine within 24 hours (DMEA 80%; TMA 86%). Oral intake of TMA increased the DMEA content of plasma and urine dose dependently, although there were large individual differences. Plasma and urinary TMA concentrations also increased, but not dose dependently. Moreover, the findings suggested the formation of endogenous TMA, little dealkylation of DMEA and TMA, and considerable first-pass metabolism.

CONCLUSIONS

Although intake of TMA reduced N-oxygenation of DMEA and TMA, total urinary DMEA values (aggregate of DMEA and its oxide DMEAO excretion) were unaffected. Thus, monitoring occupational exposure to DMEA by analysis of biological specimens is not confounded by dietary intake of TMA, provided that total urinary DMEA is monitored. Although the increased urinary and hidrotic excretion of DMEA may contribute to body odour problems, they were primarily due to TMA excretion, which is much the greater.

The N-oxidation of trimethylamine in a Jordanian population

The ability to oxidise trimethylamine (TMA) to trimethylamine N-oxide (TMAO) is distributed polymorphically within a British white population with the majority of individuals excreting greater than 90% of total urinary TMA as TMAO. The opposite extreme is characterised by a rare inborn error of TMA N-oxidation known as the fish-odour syndrome. However there is a lack of information regarding inter-individual variability in the N-oxidation of TMA in other ethnic groups. In this study the urinary excretion of TMA and TMAO was determined over a period of 24 h in 82 Jordanian subjects. A frequency distribution histogram of % of total urinary TMA excreted as TMAO revealed that the majority of subjects excreted greater than 80% of the total urinary TMA as TMAO, however eight subjects (9.7%) excreted less than 80% of the total TMA as TMAO. In a previous study of 169 white British subjects only one (0.6%) excreted less than 80% of the total TMA as TMAO. The results suggest that the prevalence of compromised ability to N-oxidise TMA may be higher in a Jordanian population than in a British population.

Fate of dimethylamine in rat and mouse

1. [U-14C]-dimethylamine hydrochloride was administered by gavage (20 mumol/kg body weight) to adult male Wistar rat and CD1 strain mouse. 2. In both species, urine was the main route of excretion with the majority of radiolabel (91%) being voided during the first day. Additional small amounts of radioactivity were detected in the 24-72 h urine (2%), in faeces (2%) and amidst exhaled air (1%), with minor amounts remaining within the carcass (1%) after 3 days. 3. Metabolism was limited to demethylation, with the majority of the compound (89% dose; 96% urinary radioactivity) being excreted unchanged.

Fate of dimethylamine in man

1. The fate of [14C]-dimethylamine was investigated following oral administration to four male volunteers. 2. The major route of excretion was urine, with 94% of the administered radioactivity being voided over 3 days (87% during the first 24 h). Small amounts (1-3%) of radioactivity were found in the faeces and expired air. 3. Metabolism was limited with only 5% being demethylated to methylamine. The remainder of the dose was excreted unchanged. 4. Pharmacokinetic studies indicated rapid (t1/2ab = 8 min) and extensive absorption (bioavailability = 82%) from the gastrointestinal tract followed by widespread distribution and a fairly prompt excretion (t1/2el = 6-7 h) with a plasma clearance of 190 ml/min.

Urinary excretion of methylamines in men with varying intake of fish from the Baltic Sea

Fish contain methylamines, especially trimethylamine N-oxide (TMAO), trimethylamine (TMA), and dimethylamine (DMA). Further, DMA may be formed TMA and TMAO. DMA is a precursor of nitrosodimethylamine (NDMA), which is a potent carcinogen. Levels of DMA, TMA, and TMAO in urine were used as indicators of the dietary exposure and in vivo formation of these amines in 44 men, representing 3 groups with different fish consumption habits. The levels of TMA (median 0.24 mmol/mol creatinine; range 0-2.7) and TMAO (median 38 mmol/mol creatinine; range 8-290) were significantly associated with the weekly intake of fish (r = .47, p = .001, and r = .53, p = .0002, respectively), while no such relation was found for DMA (median 24 mmol/mol creatinine; range 5-46). Further, urinary levels of TMA and TMAO were dependent on recent intake of fish.

The fish odour syndrome: biochemical, familial, and clinical aspects

OBJECTIVES

To study the biochemical, familial, and clinical features of the fish odour syndrome among subjects with suspected body malodour.

DESIGN

Subjects who responded to a newspaper article were screened for the fish odour syndrome by interview and biochemical tests. Families of subjects with the syndrome were tested if possible.

SETTING

St Mary's Hospital, London, and some interviews at subjects' homes.

SUBJECTS

187 subjects (28 males) with suspected body malodour, of whom 156 (19 males) underwent biochemical tests. Five families of six of the subjects with the fish odour syndrome agreed to further tests.

MAIN OUTCOME MEASURES

Amounts of trimethylamine and trimethylamine N-oxide in urine collected over 24 hours under normal dietary conditions and for eight hours after oral challenge with 600 mg trimethylamine.

RESULTS

The fish odour syndrome was diagnosed in 11 subjects: the percentage of total trimethylamine excreted in their urine samples that was oxidised to trimethylamine N-oxide was < 55% under normal dietary conditions and < 25% after oral challenge with trimethylamine (in normal subjects > 80% of trimethylamine was N-oxidised). Parents of six of the subjects with the syndrome were tested: all showed impaired N-oxidation of excreted trimethylamine (< 80%) after oral challenge, indicating that they were heterozygous carriers of the allele for the syndrome. The syndrome was associated with various psychosocial reactions including clinical depression.

CONCLUSIONS

The fish odour syndrome can be inherited in an autosomal recessive fashion. It should be considered as a possible causative factor in patients complaining of body malodour.

The exogenous origin of trimethylamine in the mouse

Although it is now generally regarded that the origin of urinary trimethylamine (TMA) is via the action of intestinal microflora on precursors such as choline, little direct evidence exists. The normal production of urinary TMA was shown to be absent in germ-free mice and greatly reduced in antibiotic-pretreated animals. Cohabitation of germ-free mice with conventional animals restored their ability to excrete TMA. This study invokes a fundamental role for the intestinal microflora in the provision of TMA from precursors within the food.

Experimental study on the metabolism of dimethylethylamine in man

Dimethylethylamine (DMEA) is an aliphatic tertiary amine, which is used as a catalyst in the mould core manufacturing. During 8 h, four healthy volunteers were exposed to four different DMEA air concentrations (10, 20, 40 and 50 mg/m3; 20 mg/m3, two subjects only). DMEA was biotransformed into dimethylethylamine N-oxide (DMEAO). On average, DMEAO, accounted for 90% of the combined amount of DMEA and DMEAO excreted into the urine. The half-lives of DMEA and DMEAO in plasma were 1.3 and 3.0 h, respectively. The urinary excretion of DMEA and DMEAO followed a two-phase pattern. The half-lives in the first phase were 1.5 h for DMEA and 2.5 h for DMEAO. In the second phase, which started about 9 h after the end of exposure, half-lives of 7 h for DMEA and 8 h for DMEAO were recorded. The combined concentration of DMEA and DMEAO, in both plasma and urine, showed an excellent correlation with the air concentration of DMEA. Thus, both urinary excretion and plasma concentration can be used for biological monitoring of exposure to DMEA. An 8-h exposure to 10 mg DMEA/m3 corresponds to a postexposure plasma concentration and 2-h postexposure urinary excretion of 4.9 mumol/l and 75 mmol/mol creatinine, respectively.

Dimethylethylamine in mould core manufacturing: exposure, metabolism, and biological monitoring

The exposure and metabolism of dimethylethylamine (DMEA) was studied in 12 mould core makers in four different foundries using the Ashland cold box technique. The mean time weighted average (TWA) full work shift DMEA exposure concentration was 3.7 mg/m3. Inhaled DMEA was excreted into urine as the original amine and as its metabolite dimethylethylamine-N-oxide (DMEAO). This metabolite made up a median of 87 (range 18-93) % of the sum of DMEA and DMEAO concentrations excreted into the urine. Occupational exposure did not significantly increase the urinary excretion of dimethylamine or methylethylamine. The data indicate half lives after the end of exposure for DMEA in urine of 1.5 hours and DMEAO of three hours. The postshift summed concentration of DMEA and DMEAO in plasma and urine is a good indicator of the TWA concentration in air during the workday, and might thus be used for biological monitoring. An air concentration of 10 mg/m3 corresponds to a urinary excretion of the summed amount of DMEA and DMEAO of 135 mmol/mol creatinine.

Poisonings from flesh of the Greenland shark Somniosus microcephalus may be due to trimethylamine

The flesh of the Greenland shark, Somniosus microcephalus, especially in fresh condition, is toxic to both dog and man. Analysis of the toxic fractions indicates the presence of large amounts of trimethylamine oxide (TMAO) but no other substances that could be directly responsible for the poisoning. The symptoms appear to be due to an acute trimethylamine (TMA) poisoning arising from intestinal reduction of TMAO to TMA. TMA stimulates contraction of the guinea-pig ileum (antagonized by atropine but not hexamethonium). Low concentrations of TMA increase contraction of the rat phrenic nerve-diaphragm preparation, whereas higher concentrations cause blockade.

The fate of trimethylamine in the rat

The metabolism and excretion of [14C]-trimethylamine has been investigated in seven strains of rat. Over 75% of the administered radioactivity was excreted via the urine within the first day, with up to 9% in the faeces. At this dose level (15 mg/kg body wt) N-oxidation was the major metabolic pathway encountered (45% excreted dose) whilst demethylation was only of minor importance (3%). The remaining compound was excreted unchanged. No significant differences were observed between the strains studied.

Controlled thermal degradation for the identification and quantification of amine N-oxides in urine

Studies of amine N-oxides in urine are important for the evaluation of occupational exposure to amines. These thermolabile compounds are difficult to handle by either gas or liquid chromatography, so a device for controlled thermal degradation has therefore been developed. It consists of a short precolumn with shut-off valves at both ends and an aluminum block for heating, and it was connected to the injection port of a gas chromatograph. After injection of amine N-oxides onto the precolumn and thermal degradation, the degradation products were allowed to enter the analytical column. Trimethylamine N-oxide (TMAO) and triethylamine N-oxide (TEAO) were investigated. Their thermal degradation patterns could be used for identification and quantification in aqueous solutions and in urine. Linear calibration graphs based on degradation product peaks (trimethylamine and O,N,N-trimethylhydroxylamine from TMAO and diethylamine and triethylamine from TEAO) were obtained for concentrations up to 500 ppm. Detection limits in aqueous solutions were 0.2 ppm (ca. 1 ng) for TMAO and 1 ppm for TEAO and the precisions were 6% and 9%, respectively. In urine, similar values were obtained for TEAO. The detection limit for TEAO corresponds to the expected concentration in urine after an 8-h exposure to air containing 0.8 mg/m3 of triethylamine.

The measurement of dimethylamine, trimethylamine, and trimethylamine N-oxide using capillary gas chromatography-mass spectrometry

We have developed a method for measuring dimethylamine (DMA), trimethylamine (TMA), and trimethylamine N-oxide (TMAO) in biological samples using gas chromatography with mass spectrometric detection. DMA, TMA, and TMAO were extracted from biological samples into acid after internal standards (labeled with stable isotopes) were added. p-Toluenesulfonyl chloride was used to form the tosylamide derivative of DMA. 2,2,2-Trichloroethyl chloroformate was used to form the carbamate derivative of TMA. TMAO was reduced with titanium(III) chloride to form TMA, which was then analyzed. The derivatives were chromatographed using capillary gas chromatography and were detected and quantitated using electron ionization mass spectrometry (GC/MS). Derivative yield, reproducibility, linearity, and sensitivity of the assay are described. The amounts of DMA, TMA, and TMAO in blood, urine, liver, and kidney from rats and humans, as well as in muscle from fishes, were determined. We also report the use of this method in a pilot study characterizing dimethylamine appearance and disappearance from blood in five human subjects after ingesting [13C]dimethylamine (0.5 mumol/kg body wt). The method we describe was much more reproducible than existing gas chromatographic methods and it had equivalent sensitivity (detected 1 pmol). The derivatized amines were much more stable and less likely to be lost as gases when samples were stored. Because we used GC/MS, it was possible to use stable isotopic labels in studies of methylamine metabolism in humans.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)