Autoxidation of synthetic isomers of triacylglycerol containing eicosapentaenoic acid

Article abstract of DOI:10.1007/s11746-997-0178-x

Several triacylglycerols (TAG) that contained eicosapentaenoic acid (EPA) were chemically synthesized and stored at 25 °C to assess the influence of TAG structure on oxidative stability and formation of oxidation products. Oxidative stability was evaluated by oxygen consumption during storage of the TAG. Autoxidation products of TAG were analyzed by high-performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS). Results showed that a 2:1 (mole/mole) mixture of trieicosapentaenoylglycerol (EEE) and tripalmitoylglycerol (PPP) was most susceptible to autoxidation. The oxidative stability of TAG that contained EPA and palmitic acid was negatively correlated with the moles of EPA in a single TAG molecule. When TAG with one EPA and two other fatty acids were oxidized, chainlength of constituent fatty acids hardly affected the oxidative stability of EPA-containing TAG molecules, except for stearic acid. HPLC and LC-MS analyses showed that monohydroperoxides were major oxidation products regardless of type of TAG. Bis- and tris-hydroperoxides were formed during autoxidation of EEE and dieicosapentaenoylpalmitoylglycerol. Monohydroperoxy epidioxides were found in all autoxidized TAG. These observations suggested that TAG structure affected the oxidation of TAG with highly unsaturated fatty acids.

Full text of DOI:10.1007/s11746-997-0178-x

Autoxidation of Synthetic Isomers of Triacylglycerol
Containing Eicosapentaenoic Acid
Yasushi Endo*, Sanae Hoshizaki, and Kenshiro Fujimoto
Department of Applied Biological Chemistry, Faculty of Agriculture, Tohoku University, Sendai 981, Japan
ABSTRACT: Several triacylglycerols (TAG) that contained triacylglycerol (TAG) structure (1–7). In previous papers
eicosapentaenoic acid (EPA) were chemically synthesized and (8,9), we found that the oxidative stability of marine oils was
stored at 25°C to assess the influence of TAG structure on ox-
idative stability and formation of oxidation products. Oxidative
stability was evaluated by oxygen consumption during storage
of the TAG. Autoxidation products of TAG were analyzed by
high-performance liquid chromatography (HPLC) and liquid
chromatography–mass spectrometry (LC–MS). Results showed
that a 2:1 (mole/mole) mixture of trieicosapentaenoylglycerol
modified by chemical and enzymatic interesterification.
These results suggested the possibility that the oxidative sta-
bilities of HUFA depend on TAG structure.
In this paper, several chemically synthesized TAG were
used to evaluate the stability of EPA-containing TAG. Oxida-
tion products, mainly hydroperoxides, were identified via
high-performance liquid chromatography (HPLC) and liquid
chromatography–mass spectrometry (LC–MS). We discuss
(EEE) and tripalmitoylglycerol (PPP) was most susceptible to au-
toxidation. The oxidative stability of TAG that contained EPA
and palmitic acid was negatively correlated with the moles of the relation between oxidative stability and structure of
EPA in a single TAG molecule. When TAG with one EPA and HUFA-containing TAG in marine oils.
two other fatty acids were oxidized, chainlength of constituent
fatty acids hardly affected the oxidative stability of EPA-contain-
MATERIALS AND METHODS
ing TAG molecules, except for stearic acid. HPLC and LC–MS
analyses showed that monohydroperoxides were major oxida-
tion products regardless of type of TAG. Bis- and tris-hydroper-
oxides were formed during autoxidation of EEE and dieicos-
apentaenoylpalmitoylglycerol. Monohydroperoxy epidioxides
were found in all autoxidized TAG. These observations sug-
gested that TAG structure affected the oxidation of TAG with
highly unsaturated fatty acids.
Materials. EPA was provided by Nihon Suisan Kaisha Ltd.
(
1
Tokyo, Japan). 1-Palmitoylglycerol, 1,2-dipalmitoylglycerol,
,3-dipalmitoylglycerol, 1,3-dilauroylglycerol, 1,3-dimyris-
toylglycerol, 1,3-distearoylglycerol, 1,3-dioleoylglycerol,
,3-dilinoleoylglycerol, glycerol, and tripalmitoylglycerol
PPP) were purchased from Sigma Chemical Co. (St. Louis,
1
(
JAOCS 74, 543–548 (1997).
MO). α-Tocopherol was purchased from Eisai Co. (Tokyo,
Japan).
KEY WORDS: Autoxidation, eicosapentaenoic acid, hy-
droperoxide, oxidative stability, triacylglycerol, triacylglycerol
structure.
Syntheses of EPA-containing TAG. The following TAG
were chemically synthesized, based on the method of Awl et
al. (10). Trieicosapentaenoylglycerol (EEE) was synthesized
by esterification of EPA with glycerol in the presence of 4-di-
methylaminopyridine and N,N′-dicyclohexylcarbodiimide as
Eicosapentaenoic acid (EPA) as well as docosahexaenoic acid catalysts. 1,2(or 2,3)-Dieicosapentaenoyl-3(or 1)-palmitoyl-
DHA), abundant in marine oils, has received much attention glycerol (EEP) and 1,3-dieicosapentaenoyl-2-palmitoylglyc-
(
because of specific physiological and nutritional effects. erol (EPE) were prepared similarly from 1,2-dipalmitoylglyc-
However, these highly unsaturated fatty acids (HUFA) are erol and 1,3-dipalmitoylglycerol. 1,3-Dilauroyl-2-eicosapen-
very susceptible to oxidation. This oxidation can produce un- taenoylglycerol (LaELa), 1,3-dimyristoyl-2-eicosapenta-
desirable flavors and harmful products during storage. Thus, enoylglycerol (MEM), 1,3-distearoyl-2-eicosapentaenoyl-
oxidation retardation during storage is required. In general, glycerol (SES), 1,3-dioleoyl-2-eicosapentaenoylglycerol
antioxidants are useful for prevention of oxidation of fats and (OEO), and 1,3-dilinoleoyl-2-eicosapentaenoylglycerol
oils during processing and storage. Unfortunately, natural an- (LiELi) were prepared from the corresponding 1,3-diacylglyc-
tioxidants, such as tocopherols, do not inhibit the oxidative erols. Synthesized TAG were purified through a Florisil col-
deterioration of HUFA-rich fish oil. It has been suggested that umn (Kanto Chemical Co. Inc., Tokyo, Japan), deactivated
the oxidative stability of vegetable oils could be affected by with 7% water, with diethyl ether/n-hexane (7:93, vol/vol). The
purity of all TAG was higher than 95% by HPLC analysis.
Autoxidation. TAG (ca. 50 mg = 600 µmol) were supple-
*
To whom correspondence should be addressed at Department of Applied
mented with α-tocopherol (0.1%) in a 10-mL test tube with a
Biological Chemistry, Faculty of Agriculture, Tohoku University, 1-1 Tsu-
tsumidori-Amamiyamachi, Aoba, Sendai 981, Japan.
W-type silicone rubber cap and stored at 25°C in the dark.
Copyright © 1997 by AOCS Press
543
JAOCS, Vol. 74, no. 5 (1997)

544
Y. ENDO ET AL.
The headspace gas (20 µL) was collected periodically and
then subjected to gas chromatography with a thermal conduc-
tivity detector to follow the oxygen uptake during storage of
TAG as described in the previous paper (9). Autoxidized
EPA-containing TAG were measured for ultraviolet ab-
sorbance at 234 nm in an ethanol solution to estimate hy-
droperoxide concentration prior to HPLC and LC–MS analy-
ses. Experiments were performed in triplicate.
Analysis of autoxidation products. HPLC and LC–MS
were used to analyze autoxidation products of synthetic EPA-
containing TAG. A reversed-phase C18 column (4.6 × 250
mm; Shiseido Co. Ltd., Tokyo, Japan) was used for HPLC
analysis. Methanol/ethanol (1:1, vol/vol) was used for EPP
and EEP as a mobile phase, whereas methanol/ethanol (4:1,
vol/vol) was used for EEE. Flow rate was 0.7 mL/min except
for EEP (0.5 mL/min). TAG and their oxidation products
were monitored at 210 nm. The relative ratio of oxidation
products was estimated by the area on HPLC chromatograms.
LC–MS was performed by the electronspray–ionization mode
in a Finnigan Mat (San Jose, CA) TSQ700 MS. The vapor-
FIG. 1. Oxygen uptake of triacylglycerols containing eicosapentaenoic
acid (EPA) and palmitic acid during storage at 25°C: ●, 1,3-dieicos-
apentaenoyl-2-palmitoylglycerol; ▲, 1,2(or2,3)-dieicosapentaenoyl-
3
(or1)-palmitoylglycerol; ■, trieicosapentaenoylglycerol/tripalmitoyl-
izer was operated at 240°C; the capillary heater was set at glycerol (2:1). (E, 20:5; P, 16:0).
2
50°C. The electron multiplier was 1000 V. High-purity ni-
trogen gas was used for the sheath and auxiliary gases, which
were set at 70 and 15 psi, respectively. The conditions of LC
were the same as those of HPLC.
RESULTS
Oxidation effect of fatty acid composition and glycerol loca-
tion. TAG that consisted of EPA and palmitic acid were stored
at 25°C, and oxygen uptake was monitored (Fig.1). A 2:1
(
mole/mole) mixture of EEE and PPP oxidized faster than
EEP and EPE. Induction periods were 5 d for the EEE/PPP
treatment and 8 d for both the EEP and EPE treatments. TAG
oxidative stability was affected by the moles of EPA in a sin-
gle TAG molecule but not by the specific position of EPA on
the glycerol backbone.
When TAG that contained EPA and palmitic acid were
stored at 25°C, induction periods were 7, 10, 16, and 17 d for a
1
1
:2 (mole/mole) mixture of EEE and PPP (EEE/PPP, 1:2), a
:1 (mole/mole) mixture of EEP and PPP (EEP/PPP, 1:1), EPP, FIG. 2. Oxygen uptake of triacylglycerols of eicosapentaenoic acid and
palmitic acid during storage at 25°C: ●, PEP; ▲, EPP; ■, EEP/PPP (1:1);
and PEP, respectively (Fig. 2). A 1:2 (mole/mole) mixture of
EEE and PPP was most rapidly autoxidized among these four
TAG samples. The oxidative stability was negatively propor-
tional to the moles of EPA present in each sample. In compar-
●, EEE/PPP (1:2). See Figure 1 for abbreviation.
ing the oxidative stability of EPP and PEP, EPA at the 2-posi- EPA all had some effects on TAG oxidative stability. TAG
tion appeared to be slightly more stable. These observations in- that consisted of one EPA and two stearic (18:0), oleic (18:1),
dicate that number and position of EPA could affect the or linoleic acids (18:2) were incubated at 25°C (Fig. 4). The
oxidative stability of EPA-containing TAG molecules.
induction period of LiELi was 12 d and was shorter than OEO
Effect of other fatty acids with EPA. Oxidative stability (16 d). OEO was the most stable, and the induction period
was evaluated for TAG that contained EPA and saturated fatty was almost the same as PEP, MEM, and LaELa in Figure 3.
acids with carbon chainlengths of 12 to 18. As shown in Fig- The variety of co-constituent fatty acids in TAG with EPA
ure 3, the induction period of LaELa, MEM, and PEP was 16 might be related in the oxidative stability as well as number
d, except for SES. In general, saturated fatty acids did not af- and position of EPA in a TAG.
fect the oxidative stability of EPA-containing TAG.
LC–MS analysis of autoxidation products. Autoxidation prod-
The number of double bonds in constituent fatty acids with ucts of EPP, EEP, and EEE were compared at the initial stage of
JAOCS, Vol. 74, no. 5 (1997)

AUTOXIDATION OF EICOSAPENTAENOIC ACID–TRIACYLGLYCEROL
545
FIG. 3. Oxygen uptake of triacylglycerols of eicosapentaenoic acid and
saturated fatty acids during storage at 25°C: ●, LaELa; ▲, MEM; ■, PEP;
, SES. (La, 12:0; M, 14:0; P, 16:0; S, 18:0; E, 20:5). See Figure 1 for
●
abbreviation.
FIG. 5. HPLC (A) and LC–MS (B) chromatograms of autoxidized tri-
eicosapentaenoylglycerol (EEE). Peak 1, unoxidized EEE; peak 2, mono-
hydroperoxides; peak 3, secondary products.
Components of complicated peaks (peak 3) on HPLC chro-
matograms were analyzed by LC–MS. LC–MS analysis indi-
cated that the peak at R 9 min of autoxidized EEE might con-
t
tain monohydroperoxy epidioxides (Fig. 5B). This identifica-
tion was based on the molecular ion peak ([M + Na], m/z
1032), which was shifted to m/z 1016, corresponding to trihy-
droxy EEE after reduction with NaBH . Similarly, monohy-
4
droperoxy epidioxides ([M + Na], m/z 940 and 986) were ob-
served at R 9 and 12 min for autoxidized EPP and EEP, re-
t
spectively. On the other hand, peak 3 of autoxidized EEP and
EEE might contain bis-hydroperoxides, which give [M + Na]
ions of m/z 986 and 1016 at R 8 and 7 min in addition to
t
monohydroperoxy epdioxides. The [M + Na] ions of bis-hy-
droperoxides of EEP and EEE were shifted to m/z 954 and
FIG. 4. Oxygen uptake of triacylglycerols of eicosapentaenoic acid and
unsaturated fatty acids during storage at 25°C: ●, SES; ▲, OEO; ■,
LiELi. (S, 18:0; O, 18:1; Li, 18:2; E, 20:5). See Figure 1 for abbreviation.
9
84, corresponding to bis-hydroxy EEP and EEE, respec-
tively, after reduction. Moreover, the [M + Na] ion (m/z 1064)
owing to tris-hydroperoxides was observed at R 5.5 min in
t
oxidation (absorbance of the ethanol solution at 234 nm = 0.16) autoxidized EEE. These results indicate that major autoxida-
by HPLC and LC–MS. Figure 5 shows HPLC and LC–MS chro- tion products of EPA-containing TAG were monohydroper-
matograms of autoxidized EEE among three EPA-containing oxides, whereas secondary products depended on varieties of
TAG molecules. Similar HPLC chromatograms as shown in Fig- EPA-containing TAG.
ure 5 were obtained for all TAG. Peak 2, next to unoxidized TAG
peak 1), was observed at a retention time (R ) of about 11 to 12 products formed during autoxidation was determined for EPP,
The ratio of monohydroperoxides occupied in oxidation
(
t
min for EPP, EEP, and EEE. Peak 2 compounds which gave [M + EEP, and EEE (Fig. 6). Monohydroperoxides accounted for
Na] ions at m/z 908, 954, and 1000 for EPP, EEP, and EEE, re- about 70 to 80% of total autoxidation products and did not
spectively, were identified as monohydroperoxides of each TAG change after the initial stage of autoxidation (absorbance
(
Fig. 5B). After reduction with NaBH , these compounds gave within the range of 0.04 to 0.16 at 234 nm). Monohydroper-
4
[M + Na] ions corresponding to the monohydroxy TAG at m/z oxides were major autoxidation products for all EPA-contain-
892, 938, and 984 for EPP, EEP, and EEE, respectively.
ing TAG, although the ratio of monohydroperoxides of autox-
JAOCS, Vol. 74, no. 5 (1997)

546
Y. ENDO ET AL.
LnLL. In our previous paper (9), we reported that TAG with
two or three HUFA were very unstable while whale oil with
HUFA preferentially distributed at the 1- or 3-position was
stable. In the present studies, we could not find any difference
in the stability of EEP and EPE. These results may be due to
the high concentration of EPA in the present studies. As al-
ready shown, TAG with high levels of EPA were so suscepti-
ble to oxidation that it would be necessary to store them at a
lower temperature or mix them with more stable TAG to eval-
uate the effect of the TAG structure. For TAG with one mole
of EPA, we found that PEP was slightly more stable than EPP.
That was in agreement with previous research (1,3).
As to the properties of coexisting fatty acids, we could not
find any marked effect of saturated fatty acids on the oxida-
tive stability of EPA-containing TAG except for stearic acid.
Wada and Koizumi (3) reported that saturated fatty acids did
not affect the oxidative rate of unsaturated fatty acids coex-
isting in TAG, while Park et al. (13–15) found that the oxida-
FIG. 6. The concentration (%) of monohydroperoxides in autoxidation tive stability of unsaturated TAG was enhanced with short-
products of triacylglycerols that contained eicosapentaenoic acid: ●,
EPP; ▲, EEP/PPP (1:1); ■, EEE/PPP (1:2). (E, 20:5; P, 16:0). See Figure 1
for abbreviation.
chain saturated fatty acids. Neff et al. (5,7) also reported that
oxidative stability of soybean oil was affected by TAG com-
position and structure. A different type of oxygen absorption
curve of SES observed in the present studies may be due to
the physical state of the TAG. In this experiment, TAG were
idized EEE was slightly lower than that of autoxidized EEP and stored at 25°C, at which SES is solid, while other TAG were
EPP. Moles of EPA esterified to a TAG molecule did not con- liquid. Among unsaturated fatty acids, linoleic acid enhanced
sistently affect the ratio of monohydroperoxides in oxidation the oxidation of EPA-containing TAG, which readily pro-
products formed during autoxidation of EPA-containing TAG.
duced free radicals during autoxidation.
The conjectured structure of hydroperoxides found in au-
toxidized EPA-containing TAG is shown in Figure 7. HPLC
and LC–MS analyses showed that major autoxidation prod-
DISCUSSION
The oxidative stability of TAG molecules with different num- ucts of EPA-containing TAG were monohydroperoxides (Fig.
bers and glycerol positions of EPA was evaluated to under- 7A,C,F), regardless of TAG structure, while secondary prod-
stand the relation between oxidative stability and structure of ucts were affected by varieties of EPA-containing TAG. The
EPA-containing TAG. Data showed that TAG structure rela- presence of monohydroperoxy epidioxides (Fig. 7B,D,G) was
tive to fatty acid composition and location could be an impor- recognized in all TAG, while bis-hydroperoxides (Fig. 7E,H)
tant factor in determining the oxidative stability of TAG that were found in the autoxidized EEP and EEE. Autoxidized
contain EPA. The oxidative stability of EPA-containing TAG EEE also might contain tris-hydroperoxides (Fig. 7I). Ya-
depended primarily on the moles of EPA in a TAG molecule. mauchi et al. (16,17) found monohydroperoxides as the pre-
The EEE TAG species were most unstable, while those con- dominant components and monohydroperoxy epdioxides as
taining one mole of EPA were most resistant to autoxidation. minor components in oxidation products of methyl eicosapen-
This result means that fish oil with high concentrations of taenoate. Frankel (18) reported that the most abundant hy-
EPA could be very susceptible to oxidation. In that regard, droperoxides formed during autoxidation of trilinoleoylglyc-
fish oil after acidolysis with oleic acid was more stable than erol and trilinolenoylglycerol were monohydroperoxides.
the corresponding oil mixed with oleic acid (8). Frankel et al. Smaller amounts of other hydroperoxides, such as monohy-
(
11) assessed the oxidative stability of synthetic TAG that droperoxy epidioxide and bis- and tris-hydroperoxides, also
consisted of linoleic acid (L) and linolenic acid (Ln) and were formed. Our results were similar. However, the analysis
found that more volatile decomposition compounds were of autoxidation products suggested that the TAG structure
formed in autoxidation of a mixture of LLL and LnLnLn than could modulate the autoxidation mechanism of EPA in TAG.
LLLn and LLnL.
The formation of monohydroperoxy epidioxides showed that
The distribution of fatty acids on glycerol also affects ox- the free-radical reaction that led to cyclization might take
idative stability. Raghuveer and Hammond (1) and Wada and place in intra-fatty acid during autoxidation of all EPA-con-
Koizumi (3) reported that TAG with unsaturated fatty acids taining TAG. On the other hand, the presence of bis- and tris-
located at the 2-position of soybean oil were more stable than hydroperoxides showed that the free-radical reaction might
those at the 1- or 3-position. On the other hand, Miyashita et take place in inter-fatty acids during autoxidation of EEP and
al. (12) found that LLnL was somewhat more unstable than EEE. Instability of EEE might be due to its susceptibility to
JAOCS, Vol. 74, no. 5 (1997)

AUTOXIDATION OF EICOSAPENTAENOIC ACID–TRIACYLGLYCEROL
547
FIG. 7. Conjectured structure of hydroperoxides formed during autoxidation of triacylglycerols of eicosapentaenoic acid.
free-radical reactions in both intra- and inter-fatty acids. ACKNOWLEDGMENTS
Hence, analyzing positional isomers of hydroperoxides and
their composition would be necessary to know the detailed
autoxidation mechanism of EPA-containing TAG.
This work was financially supported by the Fisheries Agency of
Japan. Eicosapentaenoic acid was kindly provided by Nihon Suisan
Kaisha Ltd. (Tokyo, Japan).
JAOCS, Vol. 74, no. 5 (1997)

548
Y. ENDO ET AL.
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