Allyl hexanoate

Endpoint summary

Description of key information

Allyl esters like allyl hexanoate are rapidly hydrolysed in vivo to yield allyl alcohol and the corresponding carboxylic acid (in the case of allyl hexanoate this is hexanoic acid). Allyl alcohol is oxidised to acrolein, which 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 flavour 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

ABSORPTION

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. It also occurs in simulated gastric juice and simulated intestinal fluid though at a slower rate (Butterworth et al., 1975; Grundschober, 1977; Longland et al., 1977; Silver and Murphy, 1978). 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 a,b-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 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 b-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 ω-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).

Conclusions

In oral subchronic and chronic animal studies for the three allyl esters that account for 99 % of the annual volume of flavour use (i.e. allyl hexanoate, allyl heptanoate and allyl cyclohexylpropionate), reported no observed adverse effect levels (NOAELs) are in the range from 25 to 125 mg/kg/d. The NOAELs are equivalent to daily intake levels of allyl alcohol in the range from 9 to 37 mg/kg. Exposure to levels of allyl alcohol from 9 to 37 mg/kg from allyl esters have not been associated with any evidence of hepatotoxicity or liver carcinogenicity (Lijinsky and Reuber, 1987; Carpanini et al., 1978; Damske, 1980; Hagan et al., 1967).

Allyl alcohol is oxidised 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) (Sies et al., 1983; Armstrong, 1987), hepatotoxicity will not occur. Therefore, it is very unlikely under the conditions of use as flavour ingredients that allyl esters would provide sufficient levels of allyl alcohol and its metabolite acrolein to deplete the hepatocellular concentrations of glutathione.

References

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Butterworth KR, Carpanini GB, Gaunt IF, Grasso P and Lloyd AG (1975) A new approach to the evaluation of the safety of flavouring esters. Proceedings of the BPS 54, 268.

Carpanini FMB, Gaunt IF, Hardy J, Gangoli SD, Butterworth KR and Lloyd AG (1978) Short-term toxicity of allyl alcohol in rats. Toxicology 9, 29-45.

Damske DR, Mecler FJ, Belisles RP and Weir RJ (1980) 90-Day toxicity study of allyl heptanoate in rats. Report FEMA. Unpublished.

Grundschober F (1977) Toxicological assessment of flavoring esters. Toxicology 8, 387.

Hagan EC, Hansen WH, Fitzhugh OG, Jenner PM, Jones WI and Taylor JM (1967) Food flavorings and compounds of related structure. II. Subacute and chronic Toxicity. Food and Cosmetics Toxicology 5, 141-7.

Jaeschke H, Kleinwaechter C and Wendel A (1987) The role of acrolein in allyl induced lipid peroxidation and liver cell damage in mice. Biochemical Pharmacology 36, 51-7.

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Patel JM, Wood J and Leibman KC (1980) The biotransformation of allyl alcohol and acrolein in rat liver and lung preparations. Drug Metabolism and Disposition. 8, 305.

Penttila KE, Makinen J and Lindros KO (1987) Allyl alcohol liver injury: Supression by ethanol and relation to transient glutathione depletion. Pharmacology and Toxicology 60, 340-4.

Racker R (1955) Methods in Enzymology, Vol 1, Academic Press Inc, New York, 502. Cited by Legator M and Racusen D (1959) Journal of Bacteriology 77, 120.

Reid WD (1972) Mechanisms of allyl alcohol-induced hepatic necrosis. Experientia 28, 1058-61.

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

Sies H, Brigelius R, Akerboom RPM (1983) In Functions of Glutathione: Biochemical, Physiological, Toxicological and Clinical Aspects. Intrahepatic Glutathione Status. Larson A, Holmgren A, Orrenius S, Mannervik Eds. pp 51-64. Raven Press, New York.

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.

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.