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Chapter 3 GLYCOSIDES
Glycosides are a group of compounds characterised by the fact that chemically they consist of a sugar portion (or moiety) attached by a special bond to one or more nonsugar portions. Chemically they are hydroxyls of a sugar that are capable of forming ethers with other alcohols.
Glycosides are broken down upon hydrolysis with enzymes or acids to:
1. sugar moiety = glycone
2. non-sugar moiety = aglycone / active portion. This may be a phenol, alcohol or sulphur compounds.
The bond between the two moieties may involve a phenolic hydroxly group in which case an O-glycoside is formed. Similarly carbon (C-glycosides), nitrogen (N-glycosides) or sulphur (S-glycosides) may be involved.
The linkage of the two moeities involves the transfer of a uridylyl group from uridine triphosphate to a sugar 1-phosphate. The glycoside is formed by the transfer of the sugar from uridine diphosphate to a suitable acceptor (aglycone), thus forming the glycoside [Tyler 1988].
Isolation of glycosides from plants is often difficult due to the tendency for plants to contain enzymes that bring about their own hydrolysis (Samuelsson 19921.
Glycoside distribution is widespread throughout the plant kingdom. They occur in the seeds of pulses, swollen underground roots or shoots (yams, sweet potatos), flowers and leaves.
Some may be toxic especially cyanogenic and cardiac glycosides. Cooking usually renders them non-toxic. They
are mostly soluble in water and organic solvents, though the aglycones are somewhat less soluble.
Classification of glycosides is based on the nature of the aglycone. Despite the widespread distribution of some glycoside classes we often find that the same botanical families consistently contain the same aglycone types eg.
Brassicaceae - glucosinolate Rosaceae - cyanogenic
Scrophulariaceae- cardiac, phenylpropanoid Asteraceae - phenylpropanoid, flavonoid Malvaceae - anthocyanin
In cyanogenic glycosides the element nitrogen occurs in :n the form of hydrocyanic acid , also known as prussic acid -
•H one of the most toxic of all plant compounds. A number of
v0_^_flkJ amino acids are known precursors of these glycosides.
Amygdalin from the bitter almond is derived from the aromatic amino acid phenylalanine. Amygdalin is hydrolysed in the presence of the enzyme amygdalase and water, involving a two stage process to produce glucose along with an aglycone made up of benzaldehyde (scent of bitter almonds) and odourless hydrocyanic acid.
The occurrence of cyanogenic glycosides is widespread. Amygdalin and prunasin are very common amongst plants of the Rosaceae, particularly the Prunus genus. This includes not only the bitter almond, but also the kemals of apricots, peaches, and plums. Marzipan flavour is also derived from amygdalin. Cyanogenic glycosides are found o—oiu in other food plants, including linseed and manioc - a