Allyl heptanoate

Endpoint summary

Description of key information

Allyl esters like allyl heptanoate are rapidly hydrolyzed in vivo to yield allyl alcohol and the corresponding carboxylic acid (heptanoic acid).

Allyl alcohol is oxidized to acrolein. Acrolein is primarily detoxified via glutathione conjugation, but at high levels is associated with hepatotoxicity. If the concentration of acrolein is not sufficient to deplete hepatocellular concentrations of glutathione (5 mM to 10 mM), hepatotoxicity will not occur. Therefore, it is very unlikely under the conditions of use as flavor ingredients that allyl esters would provide sufficient levels of allyl alcohol and its metabolite acrolein to deplete the hepatocellular concentrations of glutathione

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

There are no studies available in which the toxicokinetic behaviour of allyl heptanoate was investigated. However, an in vitro hydrolysis study in artificial gastric and pancreatic juice and a second in vitro enzymatic hydrolysis study in pancreatin are available for the read-across substance allyl hexanoate. In addition, the stability of allyl hexanoate was investigated in tissue homogenates of rat liver and small intestinal mucosa. Therefore, an assessment of the toxicokinetic behaviour of allyl heptanoate was conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties according to the Chapter R.7c Guidance document (ECHA, 2014) and taking into account further available information from the source substance.

Allyl heptanoate is a colourless to yellowish liquid with a molecular weight of 170.3 g/mol and a water solubility of 0.043 g/L at 20 °C.The substance has a low vapour pressure of 30.3 Pa at 25 °C and the log Pow is 3.97 at 20 °C.

 

Oral absorption

In general, the physical state, log Pow, water solubility and molecular weight suggest that the substance will be readily absorbed from the GI tract. The available data on acute oral toxicity indicate that oral absorption of the test substance occurs.

One acute oral toxicity study was performed with allyl heptanoate (Jenner, 1964).The calculated LD50 value in rats was 500 mg/kg bw with a 95% confidence level ranging from 392 to 638 mg/kg bw and 444 mg/kg with a 95% confidence level ranging from 363 to 541 mg/kg in the guinea pig.

The potential of a substance to be absorbed in the gastrointestinal (GI) tract may be influenced by chemical changes taking place in GI fluids as a result of metabolism by GI flora, by enzymes released into the GI tract or by hydrolysis. These changes will alter the physico-chemical characteristics of the substance and hence predictions based upon the physico-chemical characteristics of the parent substance may no longer apply or apply to a lesser extent (ECHA, 2014).

The ester bonds may be hydrolysed in the GI tract to form the corresponding allyl alcohol and an aliphaticacid moiety by esterases. Available data on the read-across substance allyl hexanoate indicate a hydrolysis half-life of 1120 min (= 18.7 h) in artificial gastric fluid in vitro (Longland, 1977). Hydrolysis in tissue homogenates of the small intestine and the liver was found to be much faster with a half-life of 0.096 sec and 3.96 sec, respectively (Longland, 1977). Therefore, the smaller molecules of the alcohol and aliphatic acid moieties may be absorbed faster than the parent molecule.

In conclusion, based on the available information,allyl heptanoate is predicted to undergo enzymatic hydrolysis in the GI tract and absorption of the hydrolysis products in addition to the parent substance is likely. The absorption rate of the hydrolysis products is expected to be high.

 

Dermal absorption

Allyl heptanoate is slighty water soluble (0.043 g/L) indicating a low to moderate potential for dermal absorption. Furthermore, the log Pow and molecular weight is in a range indicating dermal absorption is likely to occur (ECHA, 2014).

The dermal permeability coefficient (Kp) can be calculated from log Pow and molecular weight (MW) applying the following equation described in US EPA (2004):

log(Kp) = -2.80 + 0.66 log Pow – 0.0056 MW

This leads to a Kp of 0.109 cm/h for allyl heptanoate. Considering the water solubility the dermal flux is estimated to be 4.67 µg/cm²/h. This indicates a medium high dermal absorption potential. Furthermore, results from an acute dermal toxicity study indicate that dermal absorption occurs (Opdyke, 1974).The calculated LD50 value of allyl heptanoate was 810 mg/kg bw.

If a substance shows skin irritating or corrosive properties, damage to the skin surface may enhance penetration. An in vitro skin irritation test performed with the test substance indicated no irritating properties (Heppenheimer, 2010). Therefore, no enhanced penetration of the substance due to skin damage is expected.

Taking all the available information into account, the dermal absorption potential of allyl heptanoate is considered to be medium high.

 

Inhalation absorption

Allyl heptanoate is a liquid with low vapour pressure (30.3Pa at 25 °C), and therefore very low volatility. Consequently, under normal use and handling conditions, inhalation exposure and availability for respiratory absorption of the substance in the form of vapour, gases or mists is not significant (ECHA, 2014). The log Pow and water solubility indicate that allyl heptanoate may be absorbed across the respiratory tract epithelium to a certain extent. There is no experimental data on the effects of acute or long-term inhalation exposure to the test substance available.

 

Distribution and accumulation

The observed clinical signs and mortality in the acute oral toxicity study indicate that allyl heptanoate systemically available. As discussed under oral absorption, allyl heptanoate is expected to undergo hydrolysis in the GI tract prior to absorption. After being absorbed, the hydrolysis products are expected to be widely distributed, due to the size and the functional groups that increase the water solubility. The substances absorbed from the GI tract will be transported via the portal vein to the liver, where further metabolism can take place. Substances that are absorbed through the pulmonary alveolar membrane or through the skin enter the systemic circulation directly before transport to the liver where metabolism will take place. The substances are not expected to accumulate in adipose tissue due to the lack of lipophilic groups. 

 

Metabolism

A. Hydrolysis of allyl esters

Allyl esters are rapidly hydrolysed in vivo to yield allyl alcohol and the corresponding carboxylic acid (Silver and Murphy, 1978). Hydrolysis of these esters in vitro has been demonstrated in homogenates of the liver and intestinal mucosa. The rate of hydrolysis of straight-chain esters is approximately 100 times faster than the rate of hydrolysis of branched-chain esters (Butterworth et al., 1975). Evidence that hepatic carboxylesterases catalyse hydrolysis of allyl esters is that carboxylesterase inhibitors (triorthotolyl phosphate and S,S,S-tributylphosphotrithioate) almost completely suppress hydrolysis of a series of allyl esters in rat liver homogenate (Silver and Murphy, 1978).

 

B. Metabolism of allyl alcohol

Allyl alcohol formed by ester hydrolysis has been reported to be an excellent substrate for hepatic alcohol dehydrogenase (ADH) (Racker, 1955). In the presence of NAD+, liver ADH catalyses the rapid in vitro conversion of allyl alcohol to acrolein accompanied by the production of NADH (Serafini-Cessi, 1972; Patel et al., 1980). The highly reactive α, β-unsaturated aldehyde acrolein is metabolised via rapid conjugation with glutathione (Penttila et al., 1987) or other free thiol functions (Ohno et al., 1985). Conjugation of acrolein with glutathione may occur with or without enzyme catalysis. The glutathione adduct is subsequently reduced to the corresponding 3-hydroxypropyl glutathione conjugate which is excreted principally as the mercapturic acid or cysteine derivatives. The urine of rats given an oral dose of either allyl alcohol or the allyl esters of weak carboxylic acids (e.g. allyl acetate, allyl propionate, allyl benzoate, etc.) contained 3-hydroxypropyl mercapturic acid, N-acetyl-S-(3-hydroxypropyl)-L-cysteine, 3-hyroxypropyl- L-cysteine, and minute amounts of acrolein.

 

C. Metabolism of carboxylic acids formed by ester hydrolysis

Aliphatic acyclic carboxylic acid esters

The metabolism of linear saturated aliphatic carboxylic acid esters including heptanoic acid starts with the ready formation of coenzyme A thioesters, which are metabolised via the fatty acid b-oxidation pathway or the tricarboxylic acid cycle. Propanoic acid participates in the C1 -tetrahydrofolate pathway. Linear unsaturated carboxylic acids enter the fatty acid β-oxidation pathway regardless of the position of unsaturation in the carbon chain (Voet and Voet, 1990). As carbon chain length increases, the acids may undergo oxidation to yield diacids. These may be excreted via urine (Williams, 1959).

 

Mechanism of toxicity of allyl esters

Results of animal feeding studies for allyl alcohol and allyl esters of aliphatic acids have consistently shown that exposure to allyl alcohol may result in hepatic injury, albeit at dose levels thousands of times greater than the daily per capita intake of allyl alcohol from use of allyl esters as flavour ingredients and as components of food. The hepatotoxicity of allyl esters has been related to the rate of ester hydrolysis to form allyl alcohol and the rate of oxidation of allyl alcohol to form the recognised hepatotoxicant, acrolein. Rats that were pretreated with carboxylesterase inhibitors and then orally administered allyl esters of acetic acid, cinnamic acid, and phenoxyacetic acid exhibited decreased hepatotoxic effects as measured by marker hepatic enzyme release (L-alanine; 2-oxoglutarate transaminase; ALT) into plasma compared to controls. Pretreatment of rats with an alcohol dehydrogenase inhibitor (pyrazole) completely prevented release of the enzyme marker. These results support a mechanism for hepatotoxicity that involves enzymatic ester hydrolysis and subsequent oxidation of allyl alcohol to acrolein (Silver and Murphy, 1978).

The relative rate of hydrolysis for aliphatic allyl esters has been related to hepatotoxicity. Rats were administered allyl alcohol or equimolar doses of allyl esters at levels of 5 to 60 mg/kg allyl alcohol daily for 21 days. Livers of animals sacrificed on day 21 exhibited hepatic injury (i.e., periportal cell enlargement followed by necrosis and subsequent fibrosis with bile duct hyperplasia) to varying degrees. The severity of injury resulting from administration of an equimolar dose of straight chain esters was similar to that produced by allyl alcohol and was more marked than the damage produced by branched-chain esters (Butterworth et al., 1975). The authors concluded that rapid hydrolysis of straight-chain allyl esters formed elevated concentrations of allyl alcohol which, when oxidised to acrolein, overwhelmed the primary detoxication mechanism (i.e. glutathione conjugation) of the liver. Allyl alcohol and acrolein were formed from hydrolysis of branched-chain esters at a slower rate and were adequately detoxified by hepatic glutathione conjugation.

Acrolein formed from allyl alcohol has long been considered the hepatotoxicant which primarily causes damage to the periportal region of the liver lobule (Reid, 1972). The hepatotoxic effects of acrolein depend on the interrelationship between acrolein hepatocyte concentration and intracellular levels of glutathione, which protects against acrolein toxicity (Jaeschke et al., 1987). The concentration of acrolein is determined by its rate of formation from oxidation of allyl alcohol catalysed primarily by hepatic alcohol dehydrogenase (Belinsky et al., 1985) and its rate of removal by oxidation to acrylic acid catalysed by hepatic aldehyde dehydrogenase (Jaeschke et al., 1987) or direct conjugation with glutathione. Glutathione rapidly conjugates with acrolein until it is depleted at which time the activity of the alcohol and aldehyde dehydrogenase enzymes govern hepatocyte acrolein concentrations and subsequent hepatotoxicity (Jaeschke et al., 1987).

 

Excretion

The metabolites of allyl heptanoate such as acrolein will be conjugated with e.g. glutathione to form more water-soluble molecules and excreted via the urine. The fraction of substance that is not absorbed in the GI tract, will be excreted via the faeces.

References

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Serafini-Cessi F (1972) Conversion of allyl alcohol from acrolein by rat liver. Biochemical Journal 128, 1103.

Silver EH and Murphy SD (1978) Effect of Carboxylesterase Inhibitors on the Acute Hepatotoxicity of Esters of Allyl Alcohol. Toxicology and Applied Pharmacology 45, 377.

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Voet D and Voet JG (1990) Biochemistry. John Wiley & Sons, New York.

Williams RT (1959) Detoxication Mechanisms. pp.88-113. 2nd Edition. Chapman and Hall Ltd, London.