Potatoes contain alkaloidal glycosides and more so if sprouted. ●
Cassava root contains toxic cyanogenic glycosides – in toxic amounts in the so-called bitter varieties. ●
Many other food plants are restricted in their use due to the presence of different glycosides. ●
The problems and their solutions are discussed. ●
Table 16.1. Toxic, anti-nutritional and bitter-tasting glycosides and oligosaccharides.
Compound group or compound Example(s) Toxicity, taste, etc. A. O-Glycosides, sugar esters and oligosaccharides
In sources of food and feed
Cyanogenic glycosides Linamarin in Manihot esculenta,
Euphorbiaceae, in general widespread
in the plant kingdom (Tracheophyta and Spermatophyta)
Acute and chronic toxicity due to release of HCN. Neurotoxicity of intact glycosides discussed. Bitter taste Glycoalkaloids Chachonin and solanin in Solanum
tuberosum, Solanaceae (Angiospermae)
Corrosive to the gastrointestinal tract, acutely toxic upon absorption due to several mechanisms. Bitter taste Glycosides of organic nitriles Simmondsin in Simmondsia californica
(jojoba), Buxaceae (Angiospermae)
Causes chronic toxicity by unknown mechanism
Glycosides and sugar esters of aliphatic nitrocompounds
Miserotoxin in Astragalus spp., Fabaceae (= Leguminosae; Angiospermae)
Acutely toxic to ruminants; inhibits the citric acid cycle of cells Methylazoxymethanol (MAM)
glycosides
Cycasin in Cycas spp., Cycadaceae (Gymnospermae)
Carcinogenic Naringin In Citrus spp., especially Citrus paradisi
(grapefruit), Rutaceae (Angiospermae)
Bitter taste Oligosaccharides In seeds of several legume spp., Fabaceae
(= Leguminosae; Angiospermae)
Flatulence-producing Platyphylloside In Betula pendula, Betulaceae
(Angiospermae)
Anti-nutritional (deterrent) to several animal species
Polyphenols 2-Hydroxyarctiin in Carthamus tinctorius (safflower), Asteraceae (= Compositae;
Angiospermae)
Cathartic (laxative). Bitter taste
Ptaquiloside In Pteridium aquilinum, Polypodiaceae (Tracheophyta)
Acutely toxic and carcinogenic Saponins Triterpene or steroid saponins in Quinoa
spp., Borassus flabellifer, Glycyrrhizae glabra and Balanites spp. (Angiospermae)
Some acutely toxic, others mildly to strongly toxic. Several are bitter tasting
Vicine and convicine In Vicia faba (faba bean), Fabaceae (= Leguminosae; Angiospermae)
Acutely toxic to G6PD-deficient individuals
In medicinal and toxic plants
Carboxyatractyloside (CAT) and related compounds
CAT in Atractylis gummifera, Asteraceae (= Compositae; Angiospermae)
Acutely toxic, inhibit mitochondrial oxidative phosphorylation Cardeno- and bufodienolides ‘Digitalis glycosides’ in Digitalis spp.
(cardiac glycosides), Scrophulariaceae
(Angiospermae)
Acutely toxic to the heart
Cucurbitacins Cucurbitacin L in Citrullus colocynthis,
Cucurbitaceae (Angiospermae); some
cucurbitacins also present in food plants
Intensely bitter substances, some of which are acutely toxic Glycosides of vitamin D3 Glycosides of 1a,25-(OH)2D3 in Solanum
glaucophyllum, Solanaceae (Angiospermae)
Chronic toxicity (vitamin D intoxication – calcinosis) Ranunculin In Ranunculus and Caratocephalus spp.,
Ranunculaceae (Angiospermae)
Acutely toxic. Irritant to mucous membranes. Upon absorption affects several organs such as heart, lungs, etc. B. C-Glycosides (some also occurring as O-glycosides)
In medicinal plants
Anthraquinone, anthrone and dianthrone glycosides
Sennosides in Cassia angustifolia,
Fabaceae (= Leguminosae; Angiospermae)
Laxative effect; some compounds are drastica
C. S-Glycosides (thioglycosides)
In food and feed resources
Glucosinolates In many species within the family of Capparales (Angiospermae)
Chronic toxicity due to release of thiocyanate and other compounds. Sharp (burning) taste
even within the restricted field of toxic and anti- nutritional glycosides and oligosaccharides. Since this diversity means a broad range of different mechanisms of action, each group from Table 16.1 is commented upon below (in alphabetic order). Structure examples are also presented in a number of figures.
Anthraquinone, anthrone and dianthrone glycosides (cathartic)
The chemical class of naturally occurring anthra- noids comprises some hundreds of structurally related compounds present in many plant families within both the mono- and dicots. Laxative and/or cathartic compounds are found mainly within the genera Aloe, Cassia, Rhamnus, Rheum and Rumex. The anthranoids are oxo-, hydroxy- and hydroxy- oxo-derivatives of anthracene. Most compounds found in nature are derivatives of 9,10- anthraquinone (AQ). They occur in plants both as free anthranoids and as glycosides (Fig. 16.1). The glycosides may be hydrolysed to a limited extent in the small intestine. However, the main part is trans- ported to the colon, where microbial hydrolysis releases the aglycone. If the genuine compound
contains a 1,8-dihydroxy structure, highly reactive anthrones will be formed, directly or by reduction, which are responsible for the laxative action. The laxative effect is used in medicine; however, certain plant organs are so strong in their action that we talk about a drasticum. During the 1990s discus- sions were intense as to whether anthranoid laxa- tives present a risk of human colon cancer, but it was concluded in 2000 that several cohort studies failed to find any association between anthranoid laxative use and colorectal cancer.
A special problem to be faced in a food context is the production of cassia gum with a low content of anthraquinones and anthrones. Cassia gum is used as a food additive (in Europe designated E499), i.e. as a thickener, emulsifier, foam stabilizer, moisture reten- tion agent and/or texturizing agent in cheese, frozen dairy desserts and mixes, meat products and poultry products. Cassia gum is primarily the ground puri- fied endosperm of the seeds of Cassia tora and Cassia
obtusifolia (Family: Leguminosae). The seeds are de-
husked and de-germed by thermal mechanical treat- ment followed by milling and screening of the endosperm. Cassia gum consists mainly of high- molecular-weight (approximately 200,000–300,000) polysaccharides composed of galactomannans; the
O O OH CH3 RO OH A: R = HO O HO OH H3C B: R = HO OH OH O OH O OH OH OH O HO OH HO O OH COOH OH COOH OH Glucose-O Glucose-O (a) (b) (c)
Fig. 16.1. Anthraquinone, anthrone and dianthrone glycosides (cathartic): (a) structures A and B are the anthraqui- none glycosides frangulin A and B, respectively; (b) the anthrone glucoside (a C-glucoside) aloin; (c) a structure representing the two compounds sennoside A and B. The two latter are diastereoisomers (R,R’ and R,S’) respectively around the C9–C9’ bond which connects the two parts of the molecule.
mannose:galactose ratio is about 5:1. Other galacto- mannans used in food production include carob bean gum, guar gum and tara gum.
C. tora and C. obtusifolia seeds naturally contain
anthraquinones, the content of which is reduced from around 10,000 ppm to 250 ppm during the mechanical processing. This level is still not low enough though, which is why the ground endosperm is further purified by extraction with isopropanol to remove all anthraquinones.
The international quality standards for cassia gum all also require that the production starts with a material that contains less than 0.1 or 0.05% of seeds of Cassia occidentalis. C. occidentalis gener- ally does not grow in conjunction with C. tora or
C. obtusifolia, but still is an occasional impurity
for which the collected seeds need to be inspected.
C. occidentalis seeds are noticeably smaller and dif-
ferently shaped (flat discs instead of the longish seeds of C. tora and C. obtusifolia) and can be recognized, both as seeds and later on as splits.
The reason for this restriction is that C. occidentalis is associated with muscle toxicity. Signs of C. occi-
dentalis poisoning in general include, independent
of species affected: ataxia, muscle weakness, stub- bing, and body weight loss eventually leading to death. Skeletal muscle degeneration is the predomi- nant lesion found in the majority of animal species intoxicated with C. occidentalis; other lesions such as degenerative myopathy of myocardial muscle, congestion and pulmonary oedema, hepatic cell hypertrophy and vacuolization have also been reported.
Although many studies have been carried out to identify the toxic principles of C. occidentalis, we had to wait until 1996 when a dianthrone – an
anthraquinone-derived compound – was isolated and shown to cause the characteristic mitochon- drial myopathy produced by the plant.
Semi-refined cassia gum normally containing detectable amounts of anthraquinones has been accepted for use in pet food by several countries.
Carboxyatractyloside and related compounds
Carboxyatractyloside/atractyloside was shown in 1972 to be the toxic principle in Atractylis gummif-
era. This plant was known both as a toxic and a
medicinal plant as far back in history as about 300 bc. In about 1982, the work of a number of research groups revealed that a veterinary toxicosis resulting from the eating of young sprouts of Xanthium
strumarium is due to contents of carboxyatractyloside
(CAT). Fatal intoxications due to intake of CAT- containing plants have been described for both man and animals. CAT inhibits mitochondrial oxidative phosphorylation through inhibition of the ANT (adenine nucleotide translocase)- mediated adeno- nucleotide transport through the inner mitochondrial membrane. Closely related compounds (kaurene gly- cosides) were later isolated from hepatotoxic plants such as Wedelia asperrima and Wedelia biflora (Compositae) and Cestrum parqui (Solanaceae). For structures see Fig. 16.2.
Cardeno- and bufodienolides (cardiac glycosides)
The cardiac-active steroids grouped as the ‘cardiac glycosides’ include two subgroups called cardenolides and bufadienolides (Fig. 16.3). While
O O O OH O OH OH O O O OH HO2C O OH R CO2H 1: R = H 2: R = CO2H O OH R CO2H 3: R = H 4: R = CO2H O O O OH KO3SO KO3SO
bufadienolides have been recorded from not more than six plant families, and from the skin secretions of poisonous toads, cardenolides have a much wider although still restricted distribution within the plant kingdom. Thus, cardenolides have been found in at least 200 plant species representing 55 genera and 12 angiosperm families. Important families with respect to number of cardenolide-bearing species include asclepiadaceae, Apocynaceae, Celastraceae and Scrophulariaceae. Although William Withering
described the medicinal use of the cardenolide- containing foxglove (Digitalis purpurea,
Scrophulariaceae) in 1785, it was not until 1890 that
the toxic effects were described by Sir Thomas Fraser in his search for the sources of African arrow poisons. Some structures are shown in Fig. 16.3.
Almost every spring, some people living in the Alps in Europe who collect leaves of wild garlic (Allium ursinum) for culinary use mistakenly pick the young leaves of lily-of-the-valley (Convalaria
O RO O A B C D 1 2 12 5 8 OH 18 20 21 O O 20 A B C D A B C D lactone lactone HO
Rings A/B trans, C/D cis Rings A/B cis, C/D cis
HO 1 2 OH OCOCH3 O O O CH3 CH3 O OCH3 HO CH3 3
Fig. 16.3. Cardeno- and bufadienolides. 1, The basic skeletons of cardenolides and bufadienolides differ in the structure attached to the four-fused-ring system at C-17, with the five-membered lactone characteristic of cardenolides and the six-membered lactone of all bufadienolides. 2, The cis orientation of rings C and D is characteristic of cardenolides; furthermore a cis A/B configuration is a feature of medically important cardenolides in the plant families Apocynaceae and Scrophulariaceae, whereas trans A/B cardenolides are most widely distributed among Asclepiadaceae. 3, Oleandrin.
majalis) instead. This leads to intoxications, with
blurred vision, diarrhoea and irregular heart beat due to the content of close to 40 different cardeno- lides such as convallarin and convallatoxin in lily- of-the-valley leaves. Luckily this intoxication is seldom fatal.
The veterinary literature from Africa and North America includes many reports on the lethal effect of cardenolide-containing plants to domestic livestock, and photographic illustrations documenting that grazing cattle avoid the highly toxic milkweeds (Asclepias spp.). Likewise, intoxications of cattle and donkeys often occur when green leaves and branches from pruning of oleander (Nerium oleander) are either mixed with forage or left under a tree.
Cucurbitacins
Cucurbitacins were first characterized as the bitter compounds of cucumbers, marrows and squashes, all members of the family Cucurbitaceae. The cucur- bitacins as a group are thought to be among the bitter- est substances known to man. Thus, cucurbitacin B has been detected by taste panels in dilutions as low as 1 ppb and the glycosides of cucurbitacin E at 10 ppb. The cucurbitacins are a group of oxygenated tetracy- clic triterpenes (Fig. 16.4), some of which occur as glycosides (e.g. cucurbitacin L in Citrullus colocynthis and carnosifloside III in Hemsleya carnosiflora). While most cucurbitacins, aglycones as well as glycosides, are bitter in taste, a few are tasteless or even sweet. An example of the latter is the glycoside carnosifloside
found in H. carnosiflora (Cucurbitaceae). Certain of the cucurbitacins are not only intensely bitter but also quite toxic. Thus, the LD10 for oral intake in mice of cucurbitacin B, which is a feeding attractant to cucum- ber beetles (but a feeding deterrent to other insects), was found to be around 5 mg kg−1 BW.
Several of the species within the family
Cucurbitaceae that are used as human food natu-
rally contain cucurbitacins in amounts unacceptable to the market. However, intense domestication and breeding has resulted in cultivars low in bitter com- pounds. Thus, breeding programmes for curcurbits are constantly aware of the bitterness.
Cyanogenic glycosides
Cyanogenic compounds are compounds that upon degradation release HCN, a highly toxic clear to pale blue liquid or gas. Naturally occurring cyanogenic compounds include the following groups: (i) cyanogenic glycosides, compound a; (ii) cyanogenic lipids, compound b; (iii) cyanohydrins (= hydroxynitriles), compound c; and (iv) 2,3- epoxynitriles, compound d (Fig. 16.5). Of these compounds a, b and d are chemically relatively sta- ble, however giving rise to the formation of cyano- hydrins (compound c) as a result of their degradation. One may classify the food- related use of cyanogenic plants as follows: the plant commodity is (i) a staple food component; (ii) a minor food component (of sporadic use); (iii) a component in the production of beverages, pastry or sweets; or (iv) used in medical
O O O OH HO HO CH2OH CH3 H CH3 H CH3 H CH3 CH3 H3C H CH3 CH2 O HO HO OH CH2 O HO HO OH CH2OH O 1 2 O OH HO OH H H O OH CH3 CH3 H CH3 H CH3 CH3 H3C CH3 CH3
treatments, usually within the sector of green/alter- native medicine. Cyanogenic plants and plant parts are also grazed by animals and relatively often delib- erately included in the feedstuffs used for animal husbandry.
The HCN released from the degradation of cya- nogenic glycosides may give rise to acute as well as chronic toxicity. In general, safety regulations have been implemented in the EU and in most other industrialized countries, for foods as well as feed- stuffs. Food based on highly cyanogenic plant parts such as bitter cassava roots can be made safe (based on the levels accepted in existing standards) by processing. However, food safety authorities and the general population must be aware that changing food habits, e.g. as a result of new health trends, may cause risks of hitherto unseen exposure to cyanogenic glysosides, as in the case of an increased intake of linseed.
Cyanogenesis has been detected in prokaryotes, fungi, plants and animals. Cyanogenic constituents have been isolated from a great number of organ- isms, but the glycosides only from plants and insects. The first cyanogenic constituents to be isolated and elucidated structurally were plant gly- cosides like amygdalin (1830), sambunigrin (1928) and acacipetalin (1935). The cyanogenic glycosides, which are the most common cyanogens and
comprise around 60 structures, were recognized early as substances poisonous to wild as well as domesticated animals. An example of a cyanogenic glycoside where the sugar moiety is glucose is the cyanogenic glucoside linamarin present in cassava roots (Fig. 16.6).
Examples of total contents of cyanogenic glyco- sides in plants and plant products used for or in food are (as HCN equivalents in mg kg−1): lima bean, 200–3000; bitter almond, 300–3000; apricot and peach kernels, 100–500; flaxseed, up to 500.
The acute toxicity of HCN in man is well described. The information has been gained from fatal as well as non-fatal poisonings from both oral and respiratory exposure to HCN. Clinical symp- toms include: anxiety and excitement, rapid breath- ing, faintness, weakness, headache (pulsating), constricting sensations in the chest, facial flushing, dyspnoea, nausea, vomiting, diarrhoea, dizziness, drowsiness, confusion, convulsions, incontinence of urine and faeces, irregular respiration and coma. In the case of large lethal doses, convulsions are seen immediately, followed by coma and death.
Cyanide is absorbed in the gastrointestinal tract. It is rapidly and ubiquitously distributed through- out the body, although the highest levels are typi- cally found in the liver, lungs, blood and brain. There is no accumulation of cyanide in the blood or tissues following chronic or repeated exposure. Most cyanide is metabolized to thiocyanate in the liver by the mitochondrial sulfur transferase enzyme rhodanese and other sulfur transferases.
The toxic effects of the cyanide ion in man and animals are generally similar and are believed to result from inactivation of cytochrome oxidase, inhibition of cellular respiration and consequently histotoxic anoxia. The primary targets of cyanide toxicity are the cardiovascular, respiratory and central nervous systems.
Gly-O-C-CN R2 R1 -C-O-C-CN R2 R1 R3 O CN R2 R1 HO-C- R3-C C-CN O R2 R1 (a) (b) (c) (d)
Fig. 16.5. Classes of cyanogenic compounds: a, cya- nogenic glycoside; b, cyanogenic lipid; c, cyanohydrin (a-hydroxynitrile); d, 2,3-epoxynitrile (e.g. sarmentosin
epoxide). OH OH H H H H N N H3C CH3 H3C CH3 O O H OH HO HC N Linamarin Glucose Acetone HO
Chronic exposure to lower (non-fatal) concen- trations of HCN is known to affect the CNS of both animals and man. In production animals, cyanide is associated with syndromes affecting the CNS and gives rise to ataxia in sheep, cattle and horses grazing on another cyanogenic crop, i.e. sorghum (Sorghum bicolor (L.) Moench), which contains the cyanogenic glucoside dhurrin. Histopathological examinations of affected horses and cattle have shown spheroids in the white mat- ter of the spinal cord, mostly in the ventral funiculi, and in the cerebellar peduncles.
A restricted number of controlled long-term studies in traditional laboratory animals such as rats as well as studies in pigs and dogs have also demonstrated damage to the CNS as evidenced by findings of slower reaction time, reduced explora- tive behaviour, etc. Among a variety of neuropa- thies reported from regions of Africa with populations that consume a high level of the tuber- ous starchy root of cassava (M. esculenta Crantz), at least konzo is generally believed to be caused by cyanide from the monotonous consumption of insufficiently processed bitter cassava. The edible parts of cassava contain the two cyanogenic gluco- sides, linamarin and lotaustralin, the bitterness being – at least to a certain extent – a function of the content of these compounds. Konzo is a distinct upper motor neuron disease characterized by the sudden onset of varying degrees of symmetric, iso- lated, non-progressive spastic paraparesis.
In 2003 CAC intervened in the internationaliza- tion of cassava as a food commodity by launching a ‘Codex Standard for Sweet Cassava’. According to this, sweet cassava varieties are those that con- tain less than 50 mg HCN kg−1 (fresh weight, FW, basis). In addition to the CAC standard for sweet cassava and its standard for edible cassava flour setting a maximum level of 10 mg kg−1 FW for the total hydrocyanic acid content, Food Standards Australia New Zealand (FSANZ) is about the only authority that has published a number of risk assessments, made a proposal for a future legisla- tion and made publicly available some documented risk communications concerning the possible adverse health effects of intake of cassava roots and cassava root products. As a result of a process of risk assessment running from 2004 to 2009 and including cassava chips, bamboo shoots and other food items with a possible content of cyanogenic constituents, FSANZ thus concludes that: (i) ‘by adequate processing (peeling, slicing and cooking)
both the cyanogenic glycosides and hydrocyanic acid can be removed prior to consumption’; (ii) ‘while the current users have adequate knowledge regarding the risks associated with consumption of cassava (and bamboo shoots), more widespread use in the community would increase the public health risks’; and (iii) ‘a maximum level of 10 mg HCN/kg is considered to be necessary to confidently protect public health and safety’ when looking at ready-to-eat-cassava chips.
Glucosinolates
In 1990 already more than 100 glucosinolates were known, the structural variations being due to vari- ous side chains and the attachment of carboxylic acids as esters to the thioglucose part, glucosin- olates being thioglucosides (Fig. 16.7).
Glucosinolates are known to occur in Capparales,
Salvadorales, Violales, Euphorbiales and Tropaeolales. Reasons for interest in glucosinolates
or glucosinolate-containing plants are the various flavour, off-flavour, anti-nutritional and toxic effects, as well as positive physiological effects asso- ciated with these constituents and plants, caused by the glucosinolates themselves and byproducts thereof. The food plants of greatest interest here are different forms of cabbage, capers used as a kind of ‘spice’, and rapeseed from which we produce rape- seed oil for the production of margarine and for direct use as a food oil.
Among the most important crops containing glu- cosinolates we thus find rape, i.e. Brassica napus,