Trimethylamine

Endpoint summary

Description of key information

Bioaccumulation potential result:
Smith, Wishnok, Deen, 1994, toxicology and applied pharmacology 125, 296-308, administration of TMA-HCl intraperitoneally or intravenously to rats and investigation of the distribution, metabolism and excretion.
Zeisel and DaCosta, 1986, CANCER RESEARCH 46, 6136-6138 December 1986, investigation of methylamine excretion after ingestion of fish containing methylamines in humans.
Asatoor et al., 1965, Biochimica et biophysica acta, 111 (1965) 384-392, administration of choline chloride (intraperitoneal) or [Me14C]choline chloride (orally) or egg lecithin or trimethylamine hydrochloride (orally) or trimethylamine hydrochloride (intraperitoneal) [14C]methylamine (intraperitoneal) or L[Me-14C]Methionine (intraperitoneal) or 14C-MMA to rats (orally or intraperitoneally) and analytical verification of DMA in Urine, to test whether these substances can be transformed in DMA and excreted that way.
Al-Waiz M. et al., 1987, Toxicology 43, 117-121, (1987), investigation of the contribution of demethylation to trimethylamine metabolism in man and its quantitative importance when compared to N-oxidation. The metabolism of orally administered trimethylamine has been studied in 4 male volunteers at 2 dose levels.
Zeisel S. et al., 1989, J. Nutr. 119, 800-804, (1989), male Spraque-Dawley rats received via orogastric intubation radiolabelled choline and the uptake and metabolism of choline was investigated in the intestines. Additionally the metabolites excreted via urine were determined.
Zhang et al, 1998, Food and Chemical Toxicology, 36, 923-27, 1998, oral administration of equimolar amounts (1 mmol/kg body weight) of potential amine precursors to male Wistar rats and investigation for their potential role as precursors of dimethylamine.
Gut I. and Conney A.H., 1993, Biochem. Pharmacol., 46(2), 239-244, (1993), study of the in vitro oxidation of trimethylamine (TMA) to TMA N-oxide (TMAO) and dimethylamine (DMA) in rat liver microsomes.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100

Additional information

The metabolism, disposition and toxicokinetics of trimethylamine have been well characterized. First of all, it is important to take into account, that trimethylamine has three different sources: the diet (for example in fish), bacterial synthesis in the intestinal tract and endogenous synthesis. In particular, TMA can be build in the body during bacterial degradation of choline inside the intestinal tract but variations in choline intake that might occur as part of consumption of normal foods are not likely to significantly increase TMA and DMA production, and therefore change our exposure to precursors of nitrosamines only somewhat. The possibility of bacterial synthesis of TMA, could not be proven in a study conducted by Smith et al. in 1994, even though in these experiments the process might have been masked by the oxidation of TMA to TMAO. Normally the dietary intake of preformed DMA and TMA and endogenous production of these amines are likely to be far more significant sources. In total the knowledge of the endogenous occurrence influences mainly the categorization and classification of trimethylamine.

In rats, trimethylamine was rapidly absorbed, uniformly distributed (interestingly the VD exceeds the animal size, so it has to be stated, that TMA must accumulate in some tissues more than in others) and metabolized with the majority of radioactivity eliminated as trimethylamine-oxide (about 40 %) via urine in both sexes of rats. As metabolite mainly trimethylamine-oxide is build, via flavin-dependent monooxigenases (also responsible for the demethylation to DMA), about 20 % of the compound is excreted unchanged as trimethylamine and the remaining 40 % are eliminated as different other metabolites, including dimethylamine. TMAO and TMA don't exert any carcinogenic potential. But there exists also the possibility of TMA being primarily metabolized to DMA and then to N-Nitrosodimethylamine, which is a potent carcinogen. Nevertheless in a study conducted with trimethylamine hydrochloride in human volunteers, the amount of trimethylamine eliminated as TMAO was found to be over 95 % of the administered dose, additionally the demethylation of TMA to DMA was found not to be an important pathway; so the risk of considerable amounts of DMA, which is then in part metabolized to N-Nitrosodimethylamine is rather low. It is not possible to exclude the possibility of nitrosamine formation, but it seems rather unlikely that a significant amount of nitrosamine can be built, since less than 5 % of the dose are available as DMA for the transformation to N-Nitrosodimethylamine. The excretion of trimethylamine is conducted mainly via the urine, and only negligible amount are excreted via faeces or exhaled.

Based on the results obtained in these studies, trimethylamine may be considered to be a rapidly absorbed and as trimethylamine-oxide well eliminated substance in rats and humans. It bears the possibility to be metabolized to dimethylamine, which can in part be transformed to the carcinogenic substance N-Nitrosodimethylamine.

Discussion on bioaccumulation potential result:

Preliminary it needs to be stated, that there are three sources of trimethylamine (TMA): diet, bacterial synthesis, and endogenous synthesis.

In 1994 Smith and Coworkers investigated the metabolism and excretion of methylamines in rats. They undertook these investigations to determine in detail the tissue distribution, urinary excretion, and possible other excretion path ways after administration of TMA-HCl to rats via injection (intravenous or intraperitoneal). The radiolabeled carbon in the trimethylamine-molecule permitted the detection of the mother compound and in addition the metabolites derived from TMA-HCl.

Blood samples were taken at 15 min, 30 min, and 1, 2, 4, and 8 hr following the dose. And in conclusion the blood concentrations obtained for iv or ip doses were indistinguishable. Additionally urine samples (24 h collections), faeces samples and the exhaled air were investigated. To determine partitioning between blood and tissues, samples of blood, lung, heart, liver, kidney, intestine, and abdominal muscle were also taken for analysis. When trimethylamine hydrochloride (TMA-HCl) is administered in vivo intravenously or intraperitoneally it is readily absorbed and uniformly distributed in the body, interestingly with the volume of distribution exceeding the animal size, suggesting that it is highly concentrated at one or more locations in the body (Smith, 1994).

The metabolism of TMA occurs mainly via oxidation to TMAO. About 40% of the dose, administered to rats was excreted in the urine as TMAO, about 20 % were excreted as TMA and the remaining 40 % where other metabolites (Smith, 1994). The results indicate also a saturation of the metabolism of TMA (Smith, 1994), because with a raising dose of TMA the TMA fraction recovered in the urine raised too. Its elimination is primarily conducted via urine (96 % of the dose could be recovered in the urine after 24 hours) and only negligible excretion occurs via exhalation or via feces (Smith, 1994). For both DMA and TMA, — 1% of the dose was recovered in exhaled air as 14CO2, while a negligible amount was exhaled as the amine. These results demonstrate that both faecal and respiratory excretion of methylamines in normal rats are negligible. Retention of either the original methylamines or any carbon-based metabolites in tissues is also negligible. These findings indicate that the only important pathway for elimination of these methylamines (and their metabolites) from the body is urinary excretion.

Additionally there was no apparent bacterial synthesis of TMA, but actual bacterial synthesis might have been masked by oxidation to TMAO and/or demethylation to DMA. As TMA can also undergo reaction to NDMA (N-nitrosodimethylamine), which is a potent carcinogen, this topic needs to be discussed here as well. The results from the study do not show that this pathway is a major one, so the risk it rather small. But because it cannot be stated that this does not occur at all, this possibility needs to be taken into account when judging the carcinogenic potential of TMA.

Zeisel and DaCosta reported that DMA excretion in humans was more than 4 times greater on the day that methylamine-containing fish was eaten than on the control day. (Zeisel and DaCosta, 1986). Methylamines excretion after diet of rich methylamines content was studied in humans. Human subjects (n = 5) ingested a diet of known methylamine content for 2 days. On Day 3, they ate fish at the luncheon and dinnermeals. On Day 4, they again ate the control diet. Fish and ham were the only foods which contained DMA and TMA. A single portion of fish contained as many methylamines as were normally excreted by the human in 2 days. On the day fish was eaten, subjects consumed 956 µmol TMA per day. On the control days, subjects ingested 40.8 µmol of TMA. On the day after fish was eaten, TMA excretion remained elevated, being 9 times greater than on the control days (1.6 ± 0.3 versus 0.17 ± 0.03 µmol/24h/kg bw; mean ± SE; P < 0.01). On the day after fish was eaten, TMA excretion remained elevated, being 5 times greater than on the control days (0.86 ± 0.14 µmol/24 h/kg bw, mean ± SE). Dimethylamine excretion increased more than 4-fold (from 5.6 to 24.1 µmol/24h/kg of body weight).

The study performed by Asatoor et al, in 1965, already gives important data concerning DMA production. They investigated whether DMA is generated in the mammalian body by feeding rats (in cases also administered via injection) with either choline chloride, lecithin, [14C]methylamine, or [Me14C]choline chloride (orally) or egg lecithin or trimethylamine hydrochloride (orally) or trimethylamine hydrochloride (intraperitoneal) and then analytical verification of DMA in Urine, to test whether these substances can be transformed in DMA and excreted that way. They could prove that DMA is a product of demethylation of trimethylamine. Thus, the conversion of choline to dimethylamine takes place in two stages; first, breakdown of choline to trimethylamine by intestinal bacteria and secondly, demethylation of the latter to dimethylamine. Furthermore TMA is in part metabolized to DMA, which is then excreted mainly via urine. Additionally MMA is partly methylated to form DMA. So these results are in accordance with the results obtained by Ischiwata et al. 1984.

In 1987 Al-Waiz et al. investigated the excretion of trimethylamine hydrochloride by 4 male human volunteers after oral ingestion of 2 dose levels (300 or 600 mg free base). At both dose levels over 95% of the administered trimethylamine was excreted in the N-oxide form, confirming N-oxidation as the major route of metabolism in man. In absolute terms, the amount of free trimethylamine detected after giving the larger dose increased nearly 40-fold from control values to give 404.2 + 61.6 ~mol in the 0--8 h urine sample and imparted a fish-like odour to the urine. If demethylation was an important pathway for the degradation of trimethylamine in man then it would be expected that as the dose increased and more free trimethylamine becomes available, more dimethylamine would be produced. This clearly is not the case. Probably the majority of urinary dimethylamine is under normal circumstances of endogenous origin, presumably arising from the methylation of methylamine produced via intermediary metabolism. So the small part of ingested trimethylamine that is transformed to dimethylamine could be metabolized to nitrosamines. However, even small amounts of dimethylamine, if nitrosated, may be sufficient to elicit a potentially carcinogenic response.

Zeisel an Coworkers investigated in 1989 the fate of via orogastric intubation administered choline in the rat intestine (Spraque-Dawley). With the help of radiolabeled choline its uptake and its metabolism were investigated in the intestines. Additionally the metabolites excreted via urine were determined. They found that choline increased the excretion of TMA and TMAO and increased DMA excretion. The absorption mechanisms for choline has a high capacity (most of the choline was absorbed). At the low dose, choline-derived radiolabel was absorbed from the intestinal lumen before it reached the areas of gut colonized with bacteria. At the high dose of choline, much more label reached the colon, and they suggest that an appreciable portion of the choline-derived radiolabel that was absorbed did so via diffusion across the colon. In terms of absolute amounts, much more choline-derived radiolabel was present in the colon after the high dose. They believe that the disproportionate rise in TMA and TMAO excretion observed at the higher choline dose occurred because choline reached the bacterially colonized large intestine in appreciable quantities. The capacity for TMA oxidation must be very large, since TMAO excretion could increase over a thirtyfold range. Despite this capacity, significant amounts of TMA (approximately 10% of the TMA that must have been formed prior to conversion to TMAO) escaped this fate and was excreted intact. It has been suggested that the major precursor of DMA is the TMA formed by gut bacteria. The rate of increase in DMA excretion with increasing choline dose was different from the rate of increase in TMA excretion at the lower doses of choline. The amount of TMAO that was formed suggests that increasing amounts of TMA could have been transiently available for DMA synthesis as choline doses were increased. At the higher choline dose there was no jump in DMA excretion, but there was a large increase in TMA and TMAO excretion. This suggests that formation of DMA was not tightly linked to the availability of TMA. The data show that DMA excretion was directly related to the dose of choline administered, suggesting that choline, or a metabolite of choline, is likely to be a precursor of DMA. Previously, they found that the presence of gut bacteria was not essential for the formation and excretion of DMA in rats, while gut bacteria made a significant contribution to TMA excretion.

In summary, they have found that there is a critical concentration of choline which must be achieved within gut lumen before choline absorption processes in the small intestine are overloaded, thus allowing sufficient choline to reach the large intestine. Since it is the large intestine which is colonized with bacteria, large doses of choline are, therefore, much more likely to be converted by bacteria to TMA. The data suggest that variations in choline intake that might occur as part of consumption of normal foods are not likely to significantly increase TMA and DMA production, and therefore are not likely to greatly change our exposure to precursors of nitrosamines. Dietary intake of preformed DMA and TMA and endogenous production of these amines are likely to be far more significant sources.

In 1998, Zhang et al. investigated the excretion of DMA after administration of various related amine precursors. They found trimethylamine N-oxide (at a dose rate of 1 mmol/kg body weight) to be 20 times more efficient at providing dimethylamine than an equivalent dose of choline. Additionally less than 1 % of the choline dose was converted to dimethylamine. Slightly more of an equivalent trimethylamine dose (1.6%) was converted to dimethylamine.

So trimethylamine N-oxide is undoubtedly a significant dietary source of dimethylamine and its carries the potential of being converted to carcinogenic nitrosodimethylamine. However, other dietary sources of dimethylamine probably lie undetected amidst the myriad of anutrient chemicals within foodstuffs, and the continued excretion of small amounts of dimethylamine from antibiotic treated and germfree rats (Asatoor et al., 1967; Zeisel et al., 1985) implies that dietary components, although probably the major source, are not the only originator of dimethylamine.

In 1993, Gut et al., investigated the in vitro oxidation of trimethylamine (TMA) to TMA N-oxide (TMAO) and dimethylamine (DMA) in rat liver microsomes. In details the role of flavin-containing monooxygenase and cytochrome P450 in the N-oxidation and N-demethylation of TMA was studied. Pretreatment of rats with phenobarbital, 3-methylcholanthrene, ethanol or pregnenolone 16 alpha-carbonitrile had little or no effect on the liver microsomal metabolism of TMA to TMAO or DMA. Changing the atmosphere in the incubation vessel from 20% oxygen/80% nitrogen (air) to 100% oxygen had a selective stimulatory effect on the N-oxygenation of TMA but did not affect TMA N-demethylation. In addition, the Km for TMA N-demethylation was 5-fold higher than for the N-oxygenation reaction. The results of these studies suggest that the enzyme systems responsible for N-demethylation and N-oxygenation are different and that they are under different regulatory control. Carbon monoxide (CO/O2 = 80/20) had little or no inhibitory effect on either the N-demethylation or N-oxygenation of TMA by liver microsomes from control or pregnenolone 16 alpha-carbonitrile-treated rats. Additional studies indicated that methimazole, an inhibitor of FAD-containing monooxygenase (FMO), was a potent inhibitor of TMA oxidation. Evidence was presented that FMOs are the major enzymes responsible for N-demethylation and N-oxygenation of TMA in rat liver microsomes and that cytochrome P450 enzymes do not play a major role in the metabolism of TMA to TMAO or DMA. But since N-demethylation of tertiary amines is an unusual reaction for FMO, a possibility of another thermolabile enzyme catalyzing N-demethylation of TMA cannot be excluded entirely.