PHENOLIC LIPIDS
There are essentially two types of phenolic lipid,
those in which the side-chain is isoprenoid, as in a-tocopherol,
and those which are non-isoprenoid ("polyketide origin") but with a
normal carbon chain. The former are described in another site
part, the later are described below.
Phenolic compounds are mainly present in plants.
This heterogeneous group includes simple phenols and polyphenols as well as
their derivatives and may be classified into coumarins, quinones, and flavonoids,
by far the largest group of phenolics. Among single-ring compounds those
considered as lipids are found specifically
in some plants and contain a catechol, a resorcinol or a hydroquinone nucleus alkylated by
a normal carbon chain.
They are in general toxic and present mainly in one important plant family, the Anacardiaceae,
causing serious allergic dermatitis. This family includes the well known genera: Toxicodendron
(poison ivy, poison sumac, Japanese lacquer tree), Anacardium (cashew),
and Mangifera
(mango). Similar allergenic lipids are also found in Proteaceae (Ginkgo).
Some alkylphenols are produced by industrial synthesis to be transformed into
polyethoxylate detergents (surfactants, emulsifiers) and pesticides. Because of
their man-made origins, these alkylphenols are classified as xenobiotics. The
most frequently used are octyl- and nonylphenols, the hydrocarbon chain of eight
or nine carbon atoms is attached to the phenol ring in either the ortho, meta,
or para position. Moreover, the alkyl chains may be either linear (n-alkyl
chain), or branched.
Nonylphenol
As, in animals, they mimic the effect of
estrogens, they are part of the xenoestrogen group. Their potential ecological
and human health impact is under study.
In natural phenolic lipids, the common structure of the phenolic group and the alkyl chain is altered at
the level of the aromatic ring which can be a catechol
or a resorcinol and of the carbon chain with variations
in length and degree of unsaturation. Some parent molecules are based on a hydroquinone
instead of a catechol or resorcinol.
It has been proposed to classify the long-chain phenols into groups of phenolic
acids, dihydric, and monohydric phenols. Certain plant species contain all three
phenolic types, others only two and certain sources uniquely a single material.
An extensive review of these lipids has been released by Tyman JHP (Chem Soc
Rev 1979, 8, 499).
These phenolic lipids are also named Urushiol, from the Japanese word for
the sap found in several parts of the lacquer tree, kiurushi (Rhus
toxicodendrum, poison ivy). This name
is sometimes used for all phenolic lipids.
These substances dry in air to form lustrous varnish, they are used since
a long time in China and Japan for lacquering process. They bind to skin
and their allergenicity increases with the unsaturation and lengthening of the
side chain.
R= 13 to 17 carbon atoms, the
chain having 0 to 3 double bonds
according to the plant species.
Similar compounds but with an ester link between the
catechol nucleus and a fatty acid (plamitic, stearic, oleic acids) have been
synthesized for their lipophilic antioxidant properties. All derivatives showed
betteer radical-scavenging capacities than did a-tocopherol
and ascorbyl palmitate (Torres de Pinedo A et al., Food Chem 2007, 105, 657).
Parent compounds with an additional carboxylic group
are found in cashew
nut shell (pericarp) (Anacardium occidentalis) and in seeds of Ginkgo
tree, the anacardic acids.
All the phenolic lipids contained in the cashew nut-shell have been used to make after polymerization surface coatings, modified rubbers, ion exchange resins, water-proofing materials, and in numerous other products.
The most representative is ginkgolic acid (R = 15 carbon atoms) found in Ginkgo biloba extracts (pulp and leaves). All these compounds are strongly allergenic and was shown to damage DNA (a risk for cancer development) and is toxic to nerve cells (Ahlemeyer B et al., Eur J Pharmacol. 2001, 430) and the immune system. Pharmaceutical-grade Ginkgo extract must contain less than 5 parts per million of ginkgolic acid.
These lipids are formed of a resorcinol
(1,3-dihydroxybenzene) group
alkylated at the position 5 (-R)
with an odd-numbered carbon chain (non-isoprenoid) of various length
(C17 to C27) either saturated (alkylresorcinols) or unsaturated (alkenylresorcinols) with
1 to 6 double bonds. They are very hydrophobic molecules, especially those with
the chain exceeding 11 carbon atoms.
They were first described Ginkgo biloba (Anderson
HH et al., Proc Soc Exp Biol Med 1931, 28, 609) and later in several other Anacardiaceae
such as in cashew nut shell liquid (Anacardium occidentale) (Symes
WF et al., J Am Chem Soc 1953, 75, 4952), mango latex and peel. In
these plants, resorcinol lipids were shown to be strong vesicants causing
frequently strong allergic responses. Cardol (5-pentadecyl-1,3-resorcinol) was
shown to be the strong vesicant in cashew. Cashew nuts is the main source of
phenolic lipids for the synthesis of formaldehyde-polymers used in the
automobile industry.
Resorcinol lipids were shown to be present not only in many higher plants but also
in bacteria, in fungi, in
algae, and mosses. The occurrence of
various derivatives (ring or chain modified) have also been described. The
structures of more than 100 identified natural resorcinolic lipid homologues may
be found in 11 families of higher plants and have been reviewed by Kozubek et al. (Chem Rev 1999, 99, 1).
They are
most common in cereal grains, e.g., rye (Secale cereale), wheat (Triticum
aestivum), barley (Hordeum vulgare), and millet (Pennisetum
thyphoides) (Ross AB et al., J Agric Food Chem 2003, 51, 4111).
In the major cereals, alkylresorcinols have been reported to be present in high
levels (> 500 mg/g)
in wheat and rye and in lower amounts in barley and millet. They are found
mainly in the outer layers (bran fraction) of cereal grains. The homologue
composition also varies greatly across the species; barley has the highest
proportion of C25:0 (about 40%), rye has the highest proportion of C19:0 (about
30%), and wheat has the highest proportion of C21:0 (about 50%).
Alkenylresorcinols are also present in cereals. Their amounts vary between 0.5%
for barley, 1% for wheat, and 15% for rye. Not only the amounts but also the
compositions of the specific mixtures are different. Thus, the ratio of the
homologues C17:0/C21:0 is about 0.1 in wheat and 1.0 in rye, indicating that
this determination may be used as good markers of these cereal products (Chen
Y et al., J Agric Food Chem 2004, 52, 8242).
After epidemiological studies have associated the consumption of whole grains
products with the decreased risk of chronic diseases (heart diseases, diabetes,
some cancers), phenolic compounds found in the cereals’ bran fraction
were proposed as a possible mechanism for that protection (Slavin JL et al.,
Am J Clin Nutr 1999, 70, 459).
The occurrence of alkylresorcinols in mosses
and algae are poorly documented but mixtures of six 5-alkylresorcinol homologues
were isolated from the microalga Apatococcus constipatus. The predominant
compounds have a C19, C21, or a C23 alkyl chain (Zarnowski R et al.,
Phytochemistry 2000, 55, 975).
Alkylresorcinols were reported to have antiparasitic, anticancer, antifungal,
antimicrobial, and antioxidant effects. Their various biological activities suggest their involvement in the regulation
of cell metabolism. These activities have been comprehensively reviewed by
Kozubek A et al. (Chem Rev 1999, 99,1; Cell Biol Mol Lett 2001, 6, 351).
Extensive information on these lipids may be found in the website of Dr
A. Kozubek.
These phenolic lipids have an aromatic ring
belonging to the hydroquinone group linked to a straight carbon chain.
Embelin is a well known
species of hydroquinone lipid which has a lateral chain of 11 carbon atoms. It
was characterized in a Myrsinaceae, Ambelia
ribes, which has antifertility properties and used as anthelminthic
agent in India.
Sorgoleone is an allelochemical produced by several Sorghum
species which is a natural herbicide repressing the growth of other plants
present in their surroundings (bioherbicide) (Dayan FE, Planta 2006, 224, 339).
Several analogues with various substitutions and chain lengths are found in root
exudates. The major species is 2-hydroxy-5-methoxy-3-(8’,11’,14’-pentadecatriene)-1,4-benzoquinone.
Biosynthetic studies have shown that
the unusual fatty acid
16:3D9,12,15
synthesized in
Sorghum roots
is added to a hydroquinone, the new product being further methylated (Pan Z
et al. J Biol Chem 2007, 282, 4326). Sorgoleone as an inhibitor of
mitochondrial respiration and photosynthesis is extensiely studied for field
applications.
