Allyl 3-cyclohexylpropionate

Endpoint summary

Description of key information

Allyl esters like allyl 3-cyclohexylpropionate are rapidly hydrolyzed in vivo to yield allyl alcohol and the corresponding carboxylic acid (cyclohexane propionic 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

METABOLISM

1. Hydrolysis of Allyl Esters

Allyl esters are rapidly hydrolyzed 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 catalyze 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).

 

2. 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 catalyzes 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 metabolized 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.

 

3. Metabolism of Carboxylic Acids Formed by Ester Hydrolysis

4. Alicyclic Esters

The cyclohexanecarboxylic acids formed by hydrolysis of the allyl esters in this group are cyclohexaneacetic acid, cyclohexanepropionic acid, cyclohexanebutanoic acid, cyclohexanepentanoic acid, and cylcohexanehexanoic acid.

These acids may be metabolized via b-oxidation of the side chain to yield one of two distinct metabolites which depends on whether the side chain contains an odd or even number of carbons. Substances with an odd number of carbons in the side chain are oxidized to cyclohexanecarboxylic acid, subsequently aromatized to benzoic acid and ultimately excreted as the hippuric acid. Substances with an even number of carbons do not undergo the penultimate step of aromatization to benzoic acid, but continue to undergo b-oxidation to yield ring cleavage products, primarily dicarboxylic acids which presumably enter the citric acid cycle. In the case of cyclohexaneacetic acid, an increased excretion of succinic acid has been observed (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 flavor 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 recognized 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 oxidized 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 catalyzed primarily by hepatic alcohol dehydrogenase (Belinsky et al., 1985) and its rate of removal by oxidation to acrylic acid catalyzed 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).

In oral subchronic and chronic animal studies for the three allyl esters that account for 99 % of the annual volume of flavor use (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 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) (Sies et al., 1983; Armstrong, 1987), 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. In the publication Hagan et al. (1967) it was shown, that low doses of allyl hexanoate administered to rats by gavage (100 mg/kg bw/day) over a period of 18 weeks causes adverse effects in the liver. These effects could not be observed in a one year rat feeding study, although the animals were exposed over a much longer period and to higher daily substance concentration (2500 ppm or 214 mg/kg bw/day). Allyl alcohol is metabolized by the hepatic alcohol dehydrogenase (ADH) to acrolein and then excreted via the urine after building conjugate with glutathione (GSH). Acrolein is known to cause severe liver damage via macromolecular adduct formation. In case of bolus application (via gavage) glutathione (GSH) is rapidly exhausted and acrolein elimination is retarded. Therefore acrolein remains longer in the liver and can cause specific adverse effects. We assume, that administration of the allyl esters by feeding allows a slow acrolein formation and elimination via GHS pathway without this saturation and without causing toxic effects to liver.

 

References

Armstrong RN (1987) Enzyme catalyzed detoxication reactions: Mechanisms and steriochemistra. CRC Critical Reviews in Biochemistry 22, 39-88.

Belinsky SA, Bradford BU, Forman DT, Glassman EB, Felder MR and Thurman RG (1985) Hepatotoxicity due to allyl alcohol in deermice depends on alcohol dehydrogenase. Hepatology 5, 1179-82.

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.

Lijinsky W and Reuber MD (1987) Chronic carcinogenesis studies of acrolein and related compounds. Toxicology and Industrial Health 3, 337.

Longland RC, Shilling WH and Gangolli SD (1977) The hydrolysis of flavoring esters by artificial gastrointestinal juices and rat tissue preparations. Toxicology 8, 197.

Ohno Y, Jones TW and Ormstad K (1985) Allyl alcohol toxicity in isolated renal epithelial cells: Protective effects of low molecular weight thiols. Chemico-Biological Interactions 52, 289 -99.

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.

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