Tag Archives: phytoestrogens

Lignans: definition, chemical structure, biosynthesis, metabolism, foods

Lignans: contents in brief

What are lignans?

Lignans are a subgroup of non-flavonoid polyphenols.
They are widely distributed in the plant kingdom, being present in more than 55 plant families, where they act as antioxidants and defense molecules against pathogenic fungi and bacteria.
In humans, epidemiological and physiological studies have shown that they can exert positive effects in the prevention of lifestyle-related diseases, such as type II diabetes and cancer. For example, an increased dietary intake of these polyphenols correlates with a reduction in the occurrence of certain types of estrogen-related tumors, such as breast cancer in postmenopausal women.
In addition, some lignans have also aroused pharmacological interest. Examples are:

  • podophyllotoxin, obtained from plants of the genus Podophyllum (Berberidaceae family); it is a mitotic toxin whose derivatives have been used as chemotherapeutic agents;
  • arctigenin and tracheologin, obtained from tropical climbing plants; they have antiviral properties and have been tested in the search for a drug to treat AIDS .

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Chemical structure of lignans

Their basic chemical structure consists of two phenylpropane units linked by a C-C bond between the central atoms of the respective side chains (position 8 or β), also called β-β’ bond. 3-3′, 8-O-4′, or 8-3′ bonds are observed less frequently; in these cases the dimers are called neolignans. Hence, their chemical structure is referred to as (C6-C3)2, and they are included in the phenylpropanoid group, as well as their precursors: the hydroxycinnamic acids (see below).

Lignans
Fig. 1 – Phenylpropane unit

Based on their carbon skeleton, cyclization pattern, and the way in which oxygen is incorporated in the molecule skeleton, they can be divided into 8 subgroups: furans, furofurans, dibenzylbutanes, dibenzylbutyrolactones, dibenzocyclooctadienes, dibenzylbutyrolactols, aryltetralins and arylnaphthalenes. Furthermore, there is considerable variability regarding the oxidation level of both the propyl side chains and the aromatic rings.
They are not present in the free form in nature, but linked to other molecules, mainly as glycosylated derivatives.
Among the most common lignans, secoisolariciresinol (the most abundant one), lariciresinol, pinoresinol, matairesinol and 7-hydroxymatairesinol are found.

Note: They occur not only as dimers but also as more complex oligomers, such as dilignans and sesquilignans.

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Biosynthesis of lignans

Lignans
Fig. 2 – Lignan Biosynthesis

In this section, we will examine the synthesis of some of the most common lignans.
The pathway starts from 3 of the 4 most common dietary hydroxycinnamic acids: p-coumaric acid, sinapic acid, and ferulic acid (caffeic acid is not a precursor of this subgroup of polyphenols). Therefore, they arise from the shikimic acid pathway, via phenylalanine.
The first three reactions reduce the carboxylic group of the hydroxycinnamates to alcohol group, with formation of the corresponding alcohols, called monolignols, that is, p-coumaric alcohol, sinapyl alcohol and coniferyl alcohol. These molecules also enter the pathway of lignin biosynthesis.

  • The first step, which leads to the activation of the hydroxycinnamic acids, is catalysed by hydroxycinnamate:CoA ligases, commonly called p-coumarate:CoA ligases (EC 6.2.1.12), with formation of the corresponding hydroxycinnamate-CoAs, namely, feruloil-CoA, p- coumaroyl-CoA and sinapil-CoA.
  • In the second step, a NADPH-dependent cinnamoyl-CoA: oxidoreductase, also called cinnamoyl-CoA reductase (EC1.2.1.44) catalyzes the formation of the corresponding aldehydes, and the release of coenzyme A.
  • In the last step, a NADPH-dependent cinnamyl alcohol dehydrogenase, also called monolignol dehydrogenase (EC 1.1.1.195), catalyzes the reduction of the aldehyde group to an alcohol group, with the formation of the aforementioned monolignols.

The next step, the dimerization of monolignols, involves the intervention of stereoselective mechanisms, or, more precisely, enantioselective mechanisms. In fact, most of the plant lignans exists as (+)- or (-)-enantiomers, whose relative amounts can vary from species to species, but also in different organs on the same plant, depending on the type of reactions involved.
The dimerization can occur through enzymatic reactions involving laccases (EC 1.10.3.2). These enzymes catalyze the formation of radicals that, dimerizing, form a racemic mixture. However, this does not explain how the enantiomeric mixtures found in plants are formed. The most accepted mechanism to explain the stereospecific synthesis involves the action of the laccase and of a protein able to direct the synthesis toward one or the other of the two enantiomeric forms: the dirigent protein. The reaction scheme might be: the enzyme catalyzes the synthesis of phenylpropanoid radicals that are orientated in such a way to obtain the desired stereospecific coupling by the dirigent protein.

Lignans
Fig. 3 – (-)-Matairesinol

For example, pinoresinol synthase, consisting of laccase and dirigent protein, catalyzes the stereospecific synthesis of (+)-pinoresinol from two units of coniferyl alcohol. (+)-Pinoresinol, in two consecutive stereospecific reactions catalyzed by NADPH-dependent pinoresinol/lariciresinol reductase (EC 1.23.1.2), is first reduced to (+)-lariciresinol, and then to (-)-secoisolariciresinol. (-)-Secoisolariciresinol, in the reaction catalyzed by NAD(P)-dependent secoisolariciresinol dehydrogenase (EC 1.1.1.331) is oxidized to (-)-matairesinol.

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Metabolism of lignans by human gut microbiota

Their importance to human health is due largely to their metabolism by colonic microbiota, which carries out deglycosylations, para-dehydroxylations, and meta-demethylations without enantiomeric inversion. Indeed, this metabolization leads to the formation molecules with a modest estrogen-like activity (phytoestrogens), a situation similar to that observed with some isoflavones, such as those of soybean, some coumarins, and some stilbenes. These active metabolites are the so-called “mammalian lignans or enterolignans”, such as the aglycones of enterodiol and enterolactone, formed from secoisolariciresinol and matairesinol, respectively.
Studies conducted on animals fed diets rich in lignans have shown their presence as intact molecules, in low concentrations, in serum, suggesting that they may be absorbed as such from the intestine. These molecules exhibit estrogen-independent actions, both in vivo and in vitro, such as inhibition of angiogenesis, reduction of diabetes, and suppression of tumor growth.
Note: The term “phytoestrogen” refers to molecules with estrogenic or antiandrogenic activity, at least in vitro.

Once absorbed, they enter the enterohepatic circulation, and, in the liver, may undergo phase II reactions and be sulfated or glucuronidated, and finally excreted in the urine.

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Food rich in lignans

The richest dietary source is flaxseed (linseed), that contains mainly secoisolariciresinol, but also lariciresinol, pinoresinol and matairesinol in good quantity (for a total amount of more than 3.7 mg/100 g dry weight). They are also found in sesame seeds.

Lignans
Fig. 4 – (-)-Secoisolariciresinol

Another important source is whole grains.
They are also present in other foods, but in concentrations from one hundred to one thousand times lower than those of flaxseed. Examples are:

  • beverages, generally more abundant in red wine, followed in descending order by black tea, soy milk and coffee;
  • fruits, such as apricots, pears, peaches, strawberries;
  • among vegetables, Brassicaceae, garlic, asparagus and carrots;
  • lentils and beans.

Their presence in grains and, to a lesser extent in red wine and fruit, makes them, at least in individuals who follow a Mediterranean-style eating pattern, the main source of phytoestrogens.

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References

Andersen Ø.M., Markham K.R. Flavonoids: chemistry, biochemistry, and applications. CRC Press Taylor & Francis Group, 2006

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Heldt H-W. Plant biochemistry – 3th Edition. Elsevier Academic Press, 2005

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Satake H, Koyama T., Bahabadi S.E., Matsumoto E., Ono E. and Murata J. Essences in metabolic engineering of lignan biosynthesis. Metabolites 2015;5:270-90. doi:10.3390/metabo5020270

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-46. doi:10.3390/nu2121231

van Duynhoven J., Vaughan E.E., Jacobs D.M., Kemperman R.A., van Velzen E.J.J, Gross G., Roger L.C., Possemiers S., Smilde A.K., Doré J., Westerhuis J.A.,and Van de Wiele T. Metabolic fate of polyphenols in the human superorganism. PNAS 2011;108(suppl. 1):4531-8. doi:10.1073/pnas.1000098107

Wink M. Biochemistry of plant secondary metabolism – 2nd Edition. Annual plant reviews (v. 40), Wiley J. & Sons, Inc., Publication, 2010

Isoflavones: definition, structure and soy

Isoflavones: contents in brief

What are isoflavones?

Isoflavones are colorless polyphenols belonging to the class of flavonoids.
Unlike the majority of the other flavonoids, they have a restricted taxonomic distribution, being present almost exclusively in the Leguminosae or Fabaceae plant family, mainly in soy.
Since legumes, soy in primis, are a major part of the diet in many cultures, these flavonoids may have a great impact on human health.
They are also present in beans and broad beans, but in much lower concentrations than those found in soy and soy products.
Also red clover or meadow clover (Trifolium pratense), another member of Leguminosae family, is a good source.
Currently, they are not found in fruits and vegetables.

Together with phenolic acids, such as caffeic acid and gallic acid, and quercetin glycosides, they are the most well-absorbed polyphenols, followed by flavanones and catechins (but not gallocatechins).

In plants, some isoflavones have antimicrobial activity and are synthesized in response to attacks by bacteria or fungi; thus they act as phytoalexins.

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Chemical structure of isoflavones

While most flavonoids have B ring attached to position 2 of C ring, isoflavones have B ring attached to position 3 of C ring.

Isoflavones
Fig. 1 – Isoflavone Skeleton

Even if they are not steroids, they have structural similarities to estrogens, particularly estradiol. This confers them pseudohormonal properties, such as the ability to bind estrogen receptors; therefore, they are classified as phytoestrogens or plant estrogens. The benefits often ascribed to soy and soy products (e.g. tofu) are believed to result from the ability of isoflavones to act as estrogen mimics .
It should be underlined that the binding to estrogen receptors seems to lose strength with time, therefore their potential efficacy should not be overestimated.
In foods, they are present in four forms:

  • aglycone;
  • 7-O-glucoside;
  • 6′-O-acetyl-7-O-glucoside;
  • 6′-O-malonyl-7-O-glucoside.

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Soy isoflavones: genistein, daidzein and glycitein

Isoflavones
Fig. 2 – Isoflavones

Soy and soy products, such as soy milk, tofu, tempeh and miso, are the main source of isoflavones in the human diet.
The isoflavone content of soy and soy products varies greatly as a function of growing conditions, geographic zone, and processing; for example, in soy it ranges between 580 and 3800mg/kg fresh weight, while in soy milk it range between 30 and 175 mg/L. The most abundant isoflavones in soy and soy products are genistein, daidzein and glycitein, generally present in a concentration ratio of 1:1:0,2.; biochanin A and formononetin are other isoflavones present in less concentrations.
The 6′-O-malonyl derivatives have a bitter, unpleasant, and astringent taste; therefore they give a bad flavor to the food in which they are contained. However, being sensitive to temperature, they are often hydrolyzed to glycosides during processing, such as the production of soy milk.
The fermentation processes needed for the preparation of certain foods, such as tempeh and miso, determines in turn the hydrolysis of glycosides to aglycones, i.e. the sugar-free molecule.
Isoflavone glycosides present in soy and soy products can also be deglycosylated by β-glucosidases in the small intestine.
The aglycones are very resistant to heat.
Although many compounds present in the diet are converted by intestinal bacteria to less active molecules, other compounds are converted to molecules with increased biological activity. This is the case of isoflavones, but also of prenylflavonoids from hops (Humulus lupulus), and lignans, that are other phytoestrogens.

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Soy isoflavones and menopause

Vasomotor symptoms, such as night sweats and hot flashes, and bone loss are very common in perimenopause, also called menopausal transition, and menopause. Hormone replacement therapy (HRT) has proved to be a highly effective treatment for the prevention of menopausal bone loss and vasomotor symptoms.
The use of alternative therapies based on phytoestrogens is increased as a result of the publication of the “Women’s Health Initiative” study, that suggests that hormone replacement therapy could lead to more risks than benefits, in particular an increased risk of developing some chronic diseases.
Soy isoflavones are among the most used phytoestrogens by menopausal women, often taken in the form of isoflavone fortified foods or isoflavone supplements.
However, many studies have highlighted the lack of efficacy of soy isoflavones, and red clover isoflavones, even in large doses, in the prevention of vasomotor symptoms (hot flushes and night sweats) and bone loss during menopause.

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References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Lethaby A., Marjoribanks J., Kronenberg F., Roberts H., Eden J., Brown J. Phytoestrogens for menopausal vasomotor symptom. Cochrane Database of Systematic Reviews 2013, Issue 12. Art. No.: CD001395. doi:10.1002/14651858.CD001395.pub4

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-46. doi:10.3390/nu2121231