Diversity, lignans

Lignans represent an extremely diverse group of compounds. This is the result of both structural diversity and stereo-selective biosynthesis. One particular plant species generally makes only one enantiomer of a particular compound. The other enantiomer may be synthesized by a different species. As a consequence, it is virtually impossible to summarize the biosynthesis of lignans in general. Instead, the focus here will be on the biosynthesis of the lignan podophyllotoxin in a number of different plant species, as an illustration of the different biosynthetic routes that can be used to synthesize the same compound. 

Lignans are a well-known class of widespread natural phenolic compounds that exhibit great structural and biological diversity and are commonly found in vascular plants from various families [1]. They are present at different levels of abundance in all plant parts, including roots, rhizomes, hardwood, bark, stems, leaves, flowers, fruits and seeds [2]. Lignans are of considerable pharmacological and clinical interest and are used in the treatment of cancer and other diseases [3]. The extensive pharmaceutical use of lignans is due to their antitumor, antiviral and hepatoprotective properties as well as many other beneficial activities. 

There are also many examples that highlight the stereochemical diversity of lignan biosynthesis, as observed in a cell-free extract of Arctium lappa petiole [35], which afforded secoisolariciresinol with the opposite antipode (+) to that formed by Forsythia spp. 

Table 1 lists crude extracts (mother liquor) of the plants studied however, the lignans were generally isolated after re-extractions rather than directly from the crude extracts. Extraction and isolation details are provided in the references [68-88] given in Table 1. As seen from the table, crude extracts were prepared with a polar solvent, mostly ethanol, sometimes water, but higher diversity was obtained by working with a direct acetone extract of T. maireii twigs, considering the number of the lignans isolated. An early study of Taxus lignans by Erdtman and Tsuno reported a brief comparative examination of several Taxus woods [67].

There are abundant and diverse flavonoids with carbohydrates and lipids, alkaloids (betalain alkaloids and other alkaloids), phenols (chromones, cou-marins, lignans, quinines, and other phenolics), terpenoids (monoterpenoids, sesquiterpene lactones, triperpenoid saponins, carotenoids, and other terpenoids), and minerals as micronutritional phytochemicals in fruits and vegetables of our daily diets. Among these phytochemicals, the flavonoids have specific functionality in relation to age-related diseases such as hypertension, diabetes, cardiac infarction, cataracts, and cancer. The authors of each chapter in the first section have presented their evidence in relation to the mechanism of the preventative and therapeutic ability of the compounds. 

Flavonolignans are the most extensively studied group of non-conventional lignans, particularly because of their diverse pharmacological activities. Milk thistle (Silybum marianum), an ancient therapy for acute and chronic liver diseases, is the first known source of this group of compounds (15). Havonolignans, isolated... 

Suzuki, S. (2002) Stereochemical diversity in lignan biosynthesis and establishment of norlignan biosynthetic pathway. Wood Res., 89, 52-60. 

Suzuki, S., Umezawa, T., Shimada, M. (2002b) Stereochemical diversity in lignan biosynthesis of Arctium lappa L. Biosci. Biotechnol. Biochem., 66,1262-9. 

It has been noted that the chemical diversity of plant phenolics is as vast as the plant diversity itself. Most plant phenolics are derived directly from the shikimic acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a combination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products of each of these pathways undergo additional structural elaborations that result in a vast array of plant phenolics such as simple benzoic acid and ciimamic acid derivatives, monolig-nols, lignans and lignin, phenylpropenes, coumarins, stilbenes, flavonoids, anthocyanidins, and isollavonoids. 

A dried fruit outstanding to just be enjoyed for its soft texture, chewing, and rich taste, the prune has excellent micronutrient diversity. It s rich in both prebiotic (soluble viscous) and insoluble dietary fiber (including lignans), protein, vitamins, and several essential minerals. Prunes are notable for having a low glycemic load and so are recommended for suppressing appetite. 

Wallis, A. R A. (1998) Structural diversity in lignans and neolignans. In Lewis, N. G., and Sarkanen, S. (eds). Lignin and Lignan Biosynthesis, American Chemical Society, Washington, DC. 

It is clear that lignans have a rich diversity of biological activities, which we have only begun to understand. As further studies reveal the mechanisms of their effects, benefits to human health should follow. 

D. G. Vassao K.-W. Kim L. B. Davin N. G. Lewis, Lignans (Neoiignans) and Aiiyi/Propenyi Phenois Biogenesis, Structurai Bioiogy, and Bioiogicai/Human Heaith Considerations, in Comprehensive Natural Products Chemistry II, Voi. 1 Structural Diversity I] C. Townsend, Y. Ebizuka, Eds. L. N. Mander, Fl.-W. Liu, Series Eds. Eisevier Oxford, UK, 2009 p 817-928. 

The lignans are secondary plant metabolites biosynthetically derived from the shikimate pathway. They possess diverse structures, usually dimers of phenylpropanoid units, although compounds containing three, four or even five such units, have been reported. The number of higher oligomers identified is currently increasing. 

Possibly as a result of their diverse structures, lignans show a wide range of biological activity. For example, there is growing evidence that the consumption of foods rich in lignans can decrease the risk of contracting certain forms of cancer [4-7]. The implication of these findings... 

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