Cyanogenic glycosides, Cyanides Flashcards
Glucosides
This is the largest group of secondary metabolites.
Substances consisting of sugar part and non-sugar part, so called aglycone. These two parts are connected by a bond of ether character – by glucosidic bond.
Generally speaking, we can say that these substances usually affect blood and circulatory system.
These agents are not toxic themselves, but are easily hydrolysed in acid environment and release cyanic acid
Manioc
It is widespread in all tropic areas
Root bulbs contain starch and hydrocyanic acid, which can be removed by cooking or baking
Bulbs are used as potatoes, they are dried and flour is made of them
Sorghum
It is an annual plant, about 3 m high with minute grains containing a lot of starch
Dhurrin is mainly in germs, seeds usually don’t contain cyanic acid
Poachea family
Almond, apricot, peach
Contain glucoside called amygdalin
HCN is very poisonous, but in plants occurs in quite low concentrations
Additionally, releasing process is slow so there is not sufficient amount cumulated in one time
Lower concentration of amygdalin is found in plums, blackthorns, cherries, sour cherries and apples.
MCA
The carrier of toxicity is HCN, released from glucoside
In blood it binds to haemoglobin and methaemoglobin and forms cyanhaemoglobin and cyanmethaemoglobin respectively
Its affinity to methaemoglobin (which is higher than to haemoglobin) is used for treatment
Cyanides and cyanic acid act as neurotoxins and they also bind to several proteins damaging enzyme systems.
They decrease tissue breathing, block cytochromoxidase (HCN binds to Fe and Cu), cause tissue asphyxia (suffocation), because oxygen is not passed on to tissues, blood is oversaturated by oxygen and is of bright brick red colour
Biotransformation
Dissociation of cyanogenic glucosides goes on in two steps
First, β-D-glucopyranose is dissociated due to action of enzyme called β-D-glucosidase and α-hydroxynitrile is formed
This substance is dissociated to HCN and aldehyde or ketone due to enzyme hydroxynitrillyase
Treatment
Administration of nitrites – formation of methaemoglobin, which binds cyanides – formation of cyanmethaemoglobin – administration of thiosulphates – cyanides released from methaemoglobin react with them and form thiocyanates
Now a new antidote is tested – hydroxocobalamin – with cyanides in blood forms cyanocobalamin (vitamin B12)
Lethal doses:
In fruit containing amygdalin they are 5-25 pieces of seed for children.
Lethal dose for humans is 0,5 – 3,5 mg/kg HCN
