Glycosides

Calcinogenic glycosides:

Certain plants contain glycosides of the active metabolite of vitamin D. The metabolite is called 1, 25-dihydroxycholecalciferol or more simply 1, 25-OHD3. Consumption of glycosides of 1, 25-OHD3 by grazing animals leads to a vitamin D toxicity which causes the deposition of excessive calcium in the soft tissues (calcinosis). Vitamin D functions to regulate calcium and phosporus absorption. Its metabolite, 1,25-OHD3, controls the synthesis and functions of calcium-binding proteins in the intestinal mucosa transporting calcium after digestion from the intestines to the blood stream. Of the three rangeland plants, Cestrum diurnum, Solanum malacoxylon, and Trisetum flavescens are known to contain these glycosides. The calcinogenic glycosides of C. diurnum and T. flavescens also proved to be derivatives of vitamin D3.

Carboxyatracetylosides: Cockleburs are a herbaceous annual found in the U.S. along the shores of streams and ponds and in low-lying areas of farm fields. In its seedling stage it contains a hyperglycemic glycoside named carboxyatractyloside that can be deadly to livestock.Carboxyatractyloside is a plant growth inhibitor. It has been hypothesized that carboxyatractyloside functions in a germinating cocklebur seed to keep the other seed in the fruit capsule dormant the same year. Clinical signs of poisoning in livestock include acute depression, weakness, and convulsions, and the accompanying pathologic changes include nephrosis, gastric irritation, hepatic necrosis, and marked hypoglycemia. A current and comprehensive review on the biochemistry and toxicology of atractylosides is now available.

Coumarin glycosides: The glucoside melilotoside found in sweet clover (Melilotus alba and M. officinalis) is an ether of glucose bonded with an ester bond to coumarin. It yields the toxicant dicoumarol when exposed to specific molds. Its coumarin content gives it a distinctive sweet odor similar to vanilla. A series of wet summers led to an  epidemic of “bleeding disease” in cattle. Hay with >10 ppm dicoumarol should be viewed with caution. While furocoumarins are toxic compounds that consist of a coumarin nucleus bonded to a furan ring. Several plants contain the psoralens that are generally the precursors of furocoumarins. Molds such as Penicillium nigricans, P. jensi, and the Aspergillus metabolize the coumarin into dicoumarol. Dicoumarol is similar in structure to vitamin K. When consumed by livestock it inhibits vitamin K production. Warfarin is a synthetic toxicant derived from coumarol. It is used in rat, gopher, and ground squirrel poisons and also acts as a vitamin K inhibitor to block the blood clotting process and provoke hemorrhaging. Furocoumarins are primary photodynamic agents. They absorb long-wave ultra-violet radiation upon exposure of the skin of the affected animal to sunlight and become photoactive. They then cause cell damage by inhibiting DNA synthesis by binding pyrimidine bases and nucleic acids.

Glucosinolates: Glucosinolates are sulfur-rich, anionic natural products that upon hydrolysis by endogenous thioglucosidases called myrosinases produce several different products (e.g., isothiocyanates, thiocyanates, and nitriles). About 100 glucosinolates have been so far identified. The hydrolysis products have many different biological activities, e.g., as defense compounds and attractants. For humans these compounds function as cancer-preventing agents, biopesticides, and flavor compounds. Since the completion of the Arabidopsis genome, glucosinolate research has made significant progress, resulting in near-complete elucidation of the core biosynthetic pathway, identification of the first regulators of the pathway, metabolic engineering of specific glucosinolate profiles to study function, as well as identification of evolutionary links to related pathways. Although much has been learned in recent years, much more awaits discovery before we fully understand how and why plants synthesize glucosinolates. This may enable us to fully exploit the potential of these compounds in agriculture and medicine.

By definition from Canola Council of Canada, Canola varieties are required to contain less than 30 µmoles/g of one or any combination of the four known aliphatic glucosinolates, i.e. gluconapin, progoitrin, glucobrassicanapin, and napoleiferin, in its defatted meal. It is however noteworthy that the reduction achieved through genetic manipulations in glucosinolates contents was limited only to the glucosinolates having butenyl- and pentenyl sides chains. The contents of indolyl glucosinolates, which also occur in significant quantities in rapeseed meal remained unchanged.
Glucosinolates as such are considered to be non toxic. It is, rather, their hydrolytic products which are associated with diverse antinutritional effects. Hydrolysis of glucosinolates is by the enzyme myrosinase (Thioglycoside glycohydrolase). Like other enzymes myrosinase is heat labile. Efforts have been made to inactivate it before processing. Microwave inactivation of myrosinase has been successfully accomplished, this however, was dependent on moisture content and variety of the sample. Works dealing with the effects of different processing techniques on the nutrients and antinutrients are relatively fewer. Exposure of desolventized meal to toasting is considered necessary for glucosinolates degradation and partial removal of their 
breakdown products. The same is true for the conditioning of rapeseeds prior to oil extraction, the aim of which is, among others, to inactivate native enzymes including myrosinase. Myrosinase has been reported to have been reduced by over 80% during the conditioning, pressing, extraction and toasting.

Although the enzyme myrosinase is readily destroyed by heat, the glucosinolates are more heat resistant and are not always completely destroyed and so there is a risk that they may be hydrolyzed within the gastrointestinal tract by the gut microflora. However, glucosinolates are reported to be readily removed by extraction with hot water, dilute alkali, or organic solvent mixtures. Clearly, considerable further research is required concerning the long-term toxicity of dietary glucosinolates (and their purified derivatives) in feeds for the major cultivated fish and shrimp species.
Glucosinolate profiles have not been interpreted within a phylogenic framework and little is known regarding the processes that influence the evolution of glucosinolate diversity at a macroevolutionary scale. Several interspecific polymorphisms in glucosinolate composition have been identified. A majority of these polymorphisms are lineage-specific secondary losses of glucosinolate characters, but a gain-of-character polymorphism has a lso bee n detec ted. T he genetic basis of mo st obse rved polymorphisms appears to be regulatory. In the case of A. lyrata, geographic distribution is also shown to contribute to glucosinolate metabolic diversity. Further, evidence of gene-flow between sympatric species, parallel evolution, and the existence of genetic constraints on the evolution of glucosinolates within the Brassicaceae has been observed.

Cyanogens: The cyanogenic glycosides are a group of nitrile-containing plant secondary compounds that yield cyanide following their enzymatic breakdown (cyanogenesis). It is estimated that between 3,000 and 12,000 plant species produce cyanogenic glycosides. Known cyanogenic glycosides in plants include amygdalin, linamarin, prunasin, dhurrin, lotaustralin, and taxiphyllin. Cyanogens or cyanogenetic J-glycosides occur in several important food plants and legumes, including cassava, chickpea, kidney bean, lima bean, hyacinth bean, field pea, pigeon pea, jack bean, and the oilseed linseed. Detoxification of cyanide by the formation of thiocyanates reportedly accounts for 80% of ingested cyanide. The sulphur required for this process (possibly 1.2 g/g HCN) seems to be subsequently unavailable for protein synthesis in the animal. As the S content of sorghum forage is low, a dietary deficiency of S may be induced. This manifests itself in reduced appetite and productivity.

The analysis of cassava samples processed by wet fermentation, solid-state fermentation and sun-drying for residual cyanogens and the presence of mycotoxins shows that wet fermentation was very effective in reducing cyanogen content in bitter varieties. The cyanogenic glucoside level decreased by 88% during the fermentation process while acetone cyanohydrin was retained in the cassava. Pre-fermentation processing, which involved crushing, sun drying and milling the cassava into flour, reduced the total cyanogen levels by 40%. The process resulted in considerable reduction in the cyanogenic content of the product.

Saponins: Saponins are common in a large number of plants and plant products  that are important in human and animal nutrition. Several biological effects have been ascribed to saponins. Extensive research has been carried out into the membrane- permeabilising, immunostimulant, hypocholesterolaemic and anticarcinogenic properties of saponins and they have also been found to significantly affect growth, feed intake and reproduction in animals. Pulses and oilseeds such as kidney bean, lentil, pea, chickpea, alfalfa, soybean, groundnut, lupin, mahua and sunflower have been found to contain saponins. Although no information exists concerning their toxicity within feeds for fish or shrimp, studies with chicks have reported reduced growth and feed efficiency, and interference with the absorption of dietary lipids, cholesterol, bile acids, and vitamins A and E.

The use of alfalfa, Medicago sativa, in supplemental protein meals for swine and poultry is limited by its saponin content. Although alfalfa contains several saponins (medicagenic acid, soyasapogenol A, soyasapogenol B, lucernic acid), medicagenic acid appears to be the one responsible for its antinutritional effects. Low-saponin cultivars of alfalfa have been developed. Water washing, solvent extraction, salt solution treatment use of binding agents have been tried by different workers and were able to reduce the contents of mowrin and tannin in the mahua seed cake. Alkali and acid treatment followed by ammoniation and urea treatment has also been tried. There was only partial success in degrading the saponin to non toxic sapogenins. Certain microbes has also been tried to counteract saponins in mahua seed cake but the limited success was reported. Although water washing and soaking in salt solutions can reduce the toxic constituents it is yet to achieve the commercial feasibility.

Ranunculosides or ranoculins: Ranunculosides or ranoculins as they are also called are volatile lactones. They are glycosides which upon hydrolysis split up into ranunculine and glucose. Ranunculine is then converted into an unsaturated lactone: protoanemonine. Ranunculine, protoanemonine and anemonine have medicinal properties as well but anemonic acid does not. Protoanemonine is toxic, its derivatives are not. Ranunculosides are chemically similar to coumarins (both are lactones) but are less stable as they are not phenolic substances.

Estrogens: Phytoestrogens are plant estrogens. Phytosterols are compounds exhibiting estrogenic activity and have been found in a wide variety of food plants and legumes, including wheat, rice, chick-pea, alfalfa, lupin, groundnut, linseed and soybean. For example, compounds exhibiting estrogenic activity in soybean have been identified as isoflavones, including genistein, daidzein, and coumestrol, of which genistein (4',5,7-trihydroxyisoflavone) is the most prominent. However, although genistein is reported to be heat stable, and no dietary studies have been reported to date with fish or shrimp, these compounds are not thought to pose a serious threat to fish or shrimp health.