Toxicology, 8 (1977) 387--390 © Elsevier/North-Holland Scientific Publishers Ltd.
Letter to the Editors
TOXICOLOGICAL ASSESSMENT O F F L A V O U R I N G ESTERS
We have noted with interest the publication in Toxicology of the paper b y Longland et al. [1] dealing with the use o f in vitro hydrolysis procedures as a basis for determining the necessity for further toxicological studies of flavouring esters. It is worth reiterating that the Joint FAO/WHO Expert Committee on F o o d Additives [2] have given the opinion that if evidence can be produced to show that an ester is readily hydrolysed b y mammalian enzymes to constituents whose metabolic fate and biological interactions are fully understood, further toxicological studies may n o t be necessary. We are aware of both national and international activities relating to the regulation of flavouring substances and o f the need to provide biological information in this context. In order to avoid possible duplication of experimental effort we take this o p p o r t u n i t y to bring to y o u r attention the findings of preliminary investigations undertaken under the auspices of the International Organization o f the Flavour Industry (IOFI). The studies, which were carried o u t independently at the laboratories of CIVO/TNO, Holland [ 3 ] , were designed to establish the relative rates of enzymic hydrolysis of 26 esters of organic alcohols and acids of various structural configurations, chosen on the bases summarized in Table I. Initial investigations used a preparation of pancreatin (Merck 7132). Incubations were made under arbitrary experimental conditions o f pH, enzyme concentration and substrate concentration over 2 h at 37 ° C. The extent of hydrolysis was determined b y gas--liquid chromatography. Table II records the percentage hydrolysis of each ester under the conditions of the experiment. It will be noted that in the majority o f cases, the selected esters were hydrolysed rapidly b y pancreatin, although in several instances hydrolysis was low or undetectable. F o r this reason additional investigations were undertaken on 8 esters in the latter category using whole homogenates o f pig jejunum and liver. Incubations were again made under arbitrary conditions of pH, enzyme concentration and substrate concentration and hydrolysis after 2 h was measured as before using gas--liquid chromatography. The results are summarized in Table III. It will be seen from the foregoing that, in agreement with the findings o f Longland et al. [ 1 ] , there was wide variation in the degree of in vitro hydrolysis o f flavouring esters depending on the source of mammalian enzymes
387
00 oo
Tiglic
Acetic Butyric Hexanoic Decanoic 2-Methylbutyric
Alcohols
Alicyclic aliphatic Substituted aromatic Heterocyclic Furypropionic aliphatic Aromatic Cinnamic unsatd. aliphatic
Anthranilic N-Methylanthranilic Cyclohexylpropionic Phenylacetic
~
Short chain aliphatic Longer chain aliphatic Branched chain aliphatic Unsaturated aliphatic Aromatic
Acids
~
Secondary ~Cresyl
Citron- Dimeellyl thylbenzylcarbinyl
Substituted Phenolic Terpene Highly aromatic hindered
Saturated Branched Unsaturated Saturated Branched ethylMethyl/Isoamy~ AIiyl Isopro 2 p y l i.3-Dime'thylbutyl"~enzyl
Primary
STRUCTURAL CONSIDERATIONSIN THE SELECTION OF ESTERS FOR HYDROLYSISSTUDIES
TABLEI
TABLE II HYDROLYSIS OF FLAVOURING ESTERS BY PANCREATIN Incubations were made in 0.5 M phosphate buffer at pH 7.5 and 37 ° C. The extent of hydrolysis was determined after 2 h by gas--liquid chromatography of the ester. Ester
iso-Amyl furylpropanoate Ethyl cyclohexylpropanoate AUyl hexanoate Benzyl 2-methylbutanoate m-Cresyl phenylacetate Citronellyl acetate Benzyl phenylacetate Ethyl furylpropanoate Allyl phenylacetate iso Amyl phenylacetate Ethyl decanoate Benzyl cinnamate Methyl phenylacetate o-Cresyl acetate Citronellyl phenylacetate Benzyl acetate iso Propyl phenylacetate iso Propyl butanoate iso Amyl acetate 1,3-Dimethylbutyl acetate Ethyl acetate Benzyl tiglate Allyl tiglate Methyl anthranilate Methyl-N-methylanthranilate Dimethylbenzylearbinyl acetate
Conc. a (~1/1)
27 36 60 40 < 9 mg/1 ef 15 36 40 50 < 25 f < 15 f 18 mg]l e 100 70 < 18 70 90 180 100 100 90 < 25 ~ 100 100 15 65
GLC column used for Ester
Alcohol
Carb 20 M (2) Carb 20 M (2) Carb 400 Carb 20 M (1) Dexsil 300 Carb 400 Dexsil 300 FFAP FFAP Carb 20 M (2) Carb 400 Dexsil 300 Carb 20 M ( 1 ) Carb 20 M (1) Dexsil 300 Carb 20 M (1) Carb 20 M (2) Carb 400 Carb 400 Carb 400 Carb 400 Carb 20 M (1) Carb 400 Carb 20 M (1) FFAP Carb 20 M (2)
Carb 400 Carb 400 Carb 400 Carb 20 M (1) Dexsil 300 Carb 20 M (2) Carb 20 M (2) Carb 400 Carb 400 Carb 400 Carb 400 Carb 20 M (2) Carb 400 Carb 20 M (1) Carb 20 M (2) Carb 20 M (1) Carb 400 Carb 400 Carb 400 Carb 400 Carb 400 Carb 20 M (1) Carb 400 Carb 400 Carb 400 Carb 20 M (2)
% Hydrolysis
100 100 100 100 100 100 100 100 100 90b; 100 c 80 80 70 60 60 50 50 40 20 15 _d _d __d _d --d --d
a Concentration in incubation mixture in ulfl unless otherwise stated. b 2-Methylbutyl phenylacetate. c Amyl phenylacetate. d Not measurable. e Esters dissolved in ethanol initially and then diluted with water. f Estimated concentrations.
used. Moreover, m a n y of the esters which appear to resist the action of p a n c r e a t i n were h y d r o l y s e d readily b y fresh h o m o g e n a t e s o f m a m m a l i a n tissues. In t h e a b s e n c e of e v i d e n c e to t h e c o n t r a r y it m u s t be p r e s u m e d t h a t t h o s e e s t e r s h y d r o l y s e d r a p i d l y i n v i t r o will also b e d e g r a d e d r e a d i l y i n t h e intact animal. T h e r e is a p a u c i t y o f p u b l i s h e d i n f o r m a t i o n o n t h e i n v i v o m e t a b o l i s m o f flavouring esters by m a m m a l s . C o n s e q u e n t l y , I O F I has c o m m i s s i o n e d f u r t h e r
389
TABLE III HYDROLYSIS OF FLAVOURING ESTERS BY WHOLE HOMOGENATES OF PIG JEJUNUM AND LIVER Incubations were made in 0.5 M phosphate buffer at pH 7.5 and 37 ° C. The extent of hydrolysis was determined after 2 h by gas--liquid chromatography of the ester. Ester
Conc.
GLC column used for
% Hydrolysis
Ester
Alcohol
Jejunum
Liver
500 400 400 400 400 250 250
Carb 400 Carb 400 Carb 400 Carb 400 SE 30 SE 30 SE 30
Carb 400 Carb 400 Carb 400 Carb 400 SE 30 Cart) 400 Carb 400
100 100 100 > 99 90 20 15
_a _a __a __a _a > 99 > 99
250
SE 30
SE 30
(~1/1) iso-Amyl acetate Ethyl acetate Allyl tiglate 1,3-Dimethylbutylacetate Benzyl tiglate Methyl-N-methylanthranilate Methyl anthranilate Dimethylbenzylcarbinyl acetate
0
20
a Extent of hydrolysis was not measured in these instances.
investigations on 4 esters selected on the basis of the studies above and representing classes which are hydrolysed readily (isoamylfuryl propionate and allyl phenylacetate), relatively slowly (methyl-N-methylanthranilate) or negligibly (dimethylbenzylcarbinyl acetate) under in vitro conditions. These studies using a procedure permitting the measurement of both the intestinal absorption of esters and comparison of their relative rates of hydrolysis in the guinea pig [4] will form the subject of a future report [5]. Dr. F. Grundschober Scientific Adviser o f IOFI REFERENCES 1 R.C. Longland, W.H. Shilling and S.D. Gangolli, Toxicology, 8 (1977) 197. 2 Joint FAO/WHO Expert Committee on Food Additives, l l t h Report, Specifications for the identity and purity of food additives and their toxicological evaluation. Some flavouring substances and non-nutritive sweetening agents. W.H.O. Tech. Rep. Ser. No.383, 1968. 3 D.C. Leegwater and S. van Straten, Unpublished CIVO/TNO Reports Nos. R4319 and R4414. 4 K.R. Butterworth and D. Pelling, J. Physiol., 232 (1973) 60. 5 D. Pelling, R.C. Longland, M. Dulley and S.D. Gangolli, Toxicology, submitted.
390
Letter to the Editors
TOXICOLOGICAL ASSESSMENT O F F L A V O U R I N G ESTERS
We have noted with interest the publication in Toxicology of the paper b y Longland et al. [1] dealing with the use o f in vitro hydrolysis procedures as a basis for determining the necessity for further toxicological studies of flavouring esters. It is worth reiterating that the Joint FAO/WHO Expert Committee on F o o d Additives [2] have given the opinion that if evidence can be produced to show that an ester is readily hydrolysed b y mammalian enzymes to constituents whose metabolic fate and biological interactions are fully understood, further toxicological studies may n o t be necessary. We are aware of both national and international activities relating to the regulation of flavouring substances and o f the need to provide biological information in this context. In order to avoid possible duplication of experimental effort we take this o p p o r t u n i t y to bring to y o u r attention the findings of preliminary investigations undertaken under the auspices of the International Organization o f the Flavour Industry (IOFI). The studies, which were carried o u t independently at the laboratories of CIVO/TNO, Holland [ 3 ] , were designed to establish the relative rates of enzymic hydrolysis of 26 esters of organic alcohols and acids of various structural configurations, chosen on the bases summarized in Table I. Initial investigations used a preparation of pancreatin (Merck 7132). Incubations were made under arbitrary experimental conditions o f pH, enzyme concentration and substrate concentration over 2 h at 37 ° C. The extent of hydrolysis was determined b y gas--liquid chromatography. Table II records the percentage hydrolysis of each ester under the conditions of the experiment. It will be noted that in the majority o f cases, the selected esters were hydrolysed rapidly b y pancreatin, although in several instances hydrolysis was low or undetectable. F o r this reason additional investigations were undertaken on 8 esters in the latter category using whole homogenates o f pig jejunum and liver. Incubations were again made under arbitrary conditions of pH, enzyme concentration and substrate concentration and hydrolysis after 2 h was measured as before using gas--liquid chromatography. The results are summarized in Table III. It will be seen from the foregoing that, in agreement with the findings o f Longland et al. [ 1 ] , there was wide variation in the degree of in vitro hydrolysis o f flavouring esters depending on the source of mammalian enzymes
387
00 oo
Tiglic
Acetic Butyric Hexanoic Decanoic 2-Methylbutyric
Alcohols
Alicyclic aliphatic Substituted aromatic Heterocyclic Furypropionic aliphatic Aromatic Cinnamic unsatd. aliphatic
Anthranilic N-Methylanthranilic Cyclohexylpropionic Phenylacetic
~
Short chain aliphatic Longer chain aliphatic Branched chain aliphatic Unsaturated aliphatic Aromatic
Acids
~
Secondary ~Cresyl
Citron- Dimeellyl thylbenzylcarbinyl
Substituted Phenolic Terpene Highly aromatic hindered
Saturated Branched Unsaturated Saturated Branched ethylMethyl/Isoamy~ AIiyl Isopro 2 p y l i.3-Dime'thylbutyl"~enzyl
Primary
STRUCTURAL CONSIDERATIONSIN THE SELECTION OF ESTERS FOR HYDROLYSISSTUDIES
TABLEI
TABLE II HYDROLYSIS OF FLAVOURING ESTERS BY PANCREATIN Incubations were made in 0.5 M phosphate buffer at pH 7.5 and 37 ° C. The extent of hydrolysis was determined after 2 h by gas--liquid chromatography of the ester. Ester
iso-Amyl furylpropanoate Ethyl cyclohexylpropanoate AUyl hexanoate Benzyl 2-methylbutanoate m-Cresyl phenylacetate Citronellyl acetate Benzyl phenylacetate Ethyl furylpropanoate Allyl phenylacetate iso Amyl phenylacetate Ethyl decanoate Benzyl cinnamate Methyl phenylacetate o-Cresyl acetate Citronellyl phenylacetate Benzyl acetate iso Propyl phenylacetate iso Propyl butanoate iso Amyl acetate 1,3-Dimethylbutyl acetate Ethyl acetate Benzyl tiglate Allyl tiglate Methyl anthranilate Methyl-N-methylanthranilate Dimethylbenzylearbinyl acetate
Conc. a (~1/1)
27 36 60 40 < 9 mg/1 ef 15 36 40 50 < 25 f < 15 f 18 mg]l e 100 70 < 18 70 90 180 100 100 90 < 25 ~ 100 100 15 65
GLC column used for Ester
Alcohol
Carb 20 M (2) Carb 20 M (2) Carb 400 Carb 20 M (1) Dexsil 300 Carb 400 Dexsil 300 FFAP FFAP Carb 20 M (2) Carb 400 Dexsil 300 Carb 20 M ( 1 ) Carb 20 M (1) Dexsil 300 Carb 20 M (1) Carb 20 M (2) Carb 400 Carb 400 Carb 400 Carb 400 Carb 20 M (1) Carb 400 Carb 20 M (1) FFAP Carb 20 M (2)
Carb 400 Carb 400 Carb 400 Carb 20 M (1) Dexsil 300 Carb 20 M (2) Carb 20 M (2) Carb 400 Carb 400 Carb 400 Carb 400 Carb 20 M (2) Carb 400 Carb 20 M (1) Carb 20 M (2) Carb 20 M (1) Carb 400 Carb 400 Carb 400 Carb 400 Carb 400 Carb 20 M (1) Carb 400 Carb 400 Carb 400 Carb 20 M (2)
% Hydrolysis
100 100 100 100 100 100 100 100 100 90b; 100 c 80 80 70 60 60 50 50 40 20 15 _d _d __d _d --d --d
a Concentration in incubation mixture in ulfl unless otherwise stated. b 2-Methylbutyl phenylacetate. c Amyl phenylacetate. d Not measurable. e Esters dissolved in ethanol initially and then diluted with water. f Estimated concentrations.
used. Moreover, m a n y of the esters which appear to resist the action of p a n c r e a t i n were h y d r o l y s e d readily b y fresh h o m o g e n a t e s o f m a m m a l i a n tissues. In t h e a b s e n c e of e v i d e n c e to t h e c o n t r a r y it m u s t be p r e s u m e d t h a t t h o s e e s t e r s h y d r o l y s e d r a p i d l y i n v i t r o will also b e d e g r a d e d r e a d i l y i n t h e intact animal. T h e r e is a p a u c i t y o f p u b l i s h e d i n f o r m a t i o n o n t h e i n v i v o m e t a b o l i s m o f flavouring esters by m a m m a l s . C o n s e q u e n t l y , I O F I has c o m m i s s i o n e d f u r t h e r
389
TABLE III HYDROLYSIS OF FLAVOURING ESTERS BY WHOLE HOMOGENATES OF PIG JEJUNUM AND LIVER Incubations were made in 0.5 M phosphate buffer at pH 7.5 and 37 ° C. The extent of hydrolysis was determined after 2 h by gas--liquid chromatography of the ester. Ester
Conc.
GLC column used for
% Hydrolysis
Ester
Alcohol
Jejunum
Liver
500 400 400 400 400 250 250
Carb 400 Carb 400 Carb 400 Carb 400 SE 30 SE 30 SE 30
Carb 400 Carb 400 Carb 400 Carb 400 SE 30 Cart) 400 Carb 400
100 100 100 > 99 90 20 15
_a _a __a __a _a > 99 > 99
250
SE 30
SE 30
(~1/1) iso-Amyl acetate Ethyl acetate Allyl tiglate 1,3-Dimethylbutylacetate Benzyl tiglate Methyl-N-methylanthranilate Methyl anthranilate Dimethylbenzylcarbinyl acetate
0
20
a Extent of hydrolysis was not measured in these instances.
investigations on 4 esters selected on the basis of the studies above and representing classes which are hydrolysed readily (isoamylfuryl propionate and allyl phenylacetate), relatively slowly (methyl-N-methylanthranilate) or negligibly (dimethylbenzylcarbinyl acetate) under in vitro conditions. These studies using a procedure permitting the measurement of both the intestinal absorption of esters and comparison of their relative rates of hydrolysis in the guinea pig [4] will form the subject of a future report [5]. Dr. F. Grundschober Scientific Adviser o f IOFI REFERENCES 1 R.C. Longland, W.H. Shilling and S.D. Gangolli, Toxicology, 8 (1977) 197. 2 Joint FAO/WHO Expert Committee on Food Additives, l l t h Report, Specifications for the identity and purity of food additives and their toxicological evaluation. Some flavouring substances and non-nutritive sweetening agents. W.H.O. Tech. Rep. Ser. No.383, 1968. 3 D.C. Leegwater and S. van Straten, Unpublished CIVO/TNO Reports Nos. R4319 and R4414. 4 K.R. Butterworth and D. Pelling, J. Physiol., 232 (1973) 60. 5 D. Pelling, R.C. Longland, M. Dulley and S.D. Gangolli, Toxicology, submitted.
390
