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Flashcards in Toxins from plant foods Deck (21):

Toxins from plant (3)

Cyanogenic glycosides
Plant lectins


Cyanogenic glycosides can find in ____?
Ex ?

Lima beans / Sorghum = Dhurrin
Bitter almonds = Amygdalin / Apple seeds
Bamboo shoots / Cassava = Linamarin


Cyanogenic glycosides are stabilised by ?

conjugation with glucose residues

First step in catabolism is the hydrolysis of the glucose by b-glucosidase


the b-glucosidase in Cassava ?



Process of stabilising the Cyanogenic glycosides in cassava

Linamarin + Linamarinase
---> glucose + Acetone cyanohydrin
+ Hydroxynitrile lyase
---> Acetone + Hydrogen cyanide !!


Where to find Cyanogenic glycosides in a plant ?

compartmentalised in the plant

= Linamarinase (b-glucosidase) & Hydroxnitrile lyase
Located in chloroplasts
Located in cytoplasm

Substrate and enzyme only meet when tissue is damaged such as when eaten
--> Defence mechanism against being consumed


Cassava is a major source of carbohydrate in

Africa and South America


How to process cassava to make it safe

Enzymes act quickly after harvesting and peeling

Rapid application of heat coupled with lots of washing to remove cyanide

Slow boiling in lots of water for 30 minutes

Chopping and washing in lots of water

Fermentation to remove linamarin and cyanide


Cyanide Acute toxicity

Inhibits cellular respiration by inhibiting Cytochrome C oxidase (Mitochondrial membrane protein involved in the electron transport chain)


Cyanide Acute toxicity symptoms

Mental confusion
Muscular paralysis
Respiratory failure


Cyanide Acute Minimum lethal oral dose

0.5-3.5 mg/kg body weight


Cyanide Acute toxicity Antidotes

Hydroxocobalamin (cobalt complexes with CN-)

4-dimethylaminophenol (met-haemoglobin forming antioxidant)

Amylnitrite (haem-oxidiser and vasodilator)


Cyanide Chronic toxicity symptoms

Tropical ataxic neuropathy (TAN):
- Optical atrophy
- Ataxia
- Deafness

Tropical amblyopia:
- Atrophy of optical nerves
- Blindness


Cyanide Chronic toxicity causes

chronic low level exposure due to inadequate processing

exacerbated by poor nutrition


how Cyanide Chronic toxicity is exacerbated by poor nutrition:

slide 15


What is lectin ?

what can Plant lectins be find in?

Widely distributed carbohydrate binding proteins

Black bean / Red kidney bean
Lima bean / Soy bean
Peas / Lentils


How can lectins be destroyed?

Lectins can be destroyed by cooking


What health effects can lectins cause if ingested?

lectins pass through the digestive system intact
--> Cause damage to gut epithelial cells and reduced nutrient uptake and interfere with immunity
--> Systemic effects include disruption of lipid, protein and carbohydrate metabolism leading to organ atrophy and failure

Most are chronic antinutrients; some are acutely toxic


Most toxic lectin is
can find in ?
How much to cause lethal effects?
Health effects?

ricin ; found in Castor bean

Five beans are enough to kill an adult

Causes diarrhoea, vomiting, abdominal cramps, internal bleeding, liver, kidney and circulatory failure

LD50 = 1-20 mg/kg body weight


Ricin structure and function

The structure composed by:
Ricin A chain - deactivates ribosomes by hydrolysis –-> inhibits protein synthesis

& Ricin B chain which has cell surface Oligosaccharide receptors


Concern about ricin

bioterrorist concern

ricin-laced letters has been sent US former President Obama by home-grown looneys on two occasions

in 1978, KGB used an umbrella containing a ricin pellet in it’s tip to Assassinate the Bulgarian dissident, Markov, on Waterloo Bridge