Glycosides

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Glycosides
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Glycosides

• Secondary products formed from one or more sugar
  molecules added to a non-sugar biologically
  active molecule
• Sugar most commonly found is glucose
• Non-sugar molecule known as the aglycone and may
  be
• amino acid derivatives
• steroids
• triterpenes

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Glycosides

• Most common types
• Saponins
• Cardiac Glycosides (Cardenolides)
• Cyanogenic Glycosides
• Anthraquinone Glycosides
• Glucosinolates - technically not glycosides
• Most glycosides function as deterrents to
  herbivores

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Saponins

• Terpene glycosides
• Steroid glycosides
• Steroid-alkaloid glycosides
• Triterpene glycosides
• May also occur as the aglycone and are called
  sapogenins

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Properties of Saponins

• Combination of hydrophobic terpene with a
  hydrophillic sugar produces cmpds that act as
  surfactants (detergents)
• When agitated in water, saponins form a soapy
  lather

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Effects of saponins on animals

• Variable effects
• Highly toxic to fish - used as fish poison
• Not significantly toxic to mammals
• Have a bitter acrid taste and cause severe
  gastric irritation if ingested
• Implicated in livestock poisoning - alfalfa
• Will hemolyze red blood cells if injected into
  blood stream - because of the detergent properties

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Useful saponins

• Saponins from yam - Disocorea spp. are the source
  of steroids used for human hormones
• Birth control pills - most synthetic but some
  still use natural hormones
• Anti-inflammatory steroids
• Licorice - Glycyrrhiza glabra produces saponins
  that possibly help gastric ulcers because they
  suppress prostoglandins

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Cardiac glycosides

• Structurally similar to saponins with similar
  detergent properties
• Distinguished by presence of a lactone ring and
  rare sugars
• Wide distribution - found in over 200 species (55
  genera in 12 families)
• Best known are digitoxin and digoxin from
  Digitalis (foxglove) 30 glycosides

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Digitoxin
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Sugars in digitoxin

• 2 molecules of digitose
• 1 molecule of 1-acetyl digitose
• 1 molecule of glucose

Digitose
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Action of Cardiac Glycosides

• In medicinal doses, cardiac glycosides increase
  the contractions of heart and the force of the
  heart beat
• Increases cardiac output
• Improved circulation, decreases edema, and
  increases kidney output
• Most effective for congestive heart failure
• Toxic doses cause arrhythmias or even cardiac
  arrest fine line between medicinal and toxic

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Poisonous Plants with Cyanogenic Glycosides

• Foxglove - Digitalis purpurea
• Oleander - Nerium oleander
• Lily of the Valley - Convallaria sp.
• Milkweed - Asclepias spp

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Oleander
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Oleandrin
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Milkweeds

• Another common source of glycosides are milkweeds
  - produce latex rich in cardiac glycosides
• Milkweed leaves host for monarch butterfly larvae
• Glycosides sequestered in body of larvae and
  retained through metamorphosis and present in
  butterfly
• Butterflies toxic to birds that feed on them
  causing vomiting - birds avoid butterflies

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Cyanogenic glycosides

• About 150 spp of plants produce glycosides that
  release hydrogen cyanide - HCN
• Most cyanogenic glycosides are derived from 4
  amino acids (phenylalanine, tyrosine, valine, or
  isoleucine) or from nicotinic acid
• Cyanogenic glycosides are not toxic by themselves
  but when a plant is damaged by an herbivore,
  glycoside is broken down by enzymes and HCN is
  released

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Breakdown of cyanogenic glycosides

• Two enzymes involved
• b-glycosidase releases the sugars
• Hydroxynitrile lyase releases HCN
• Does not occur in intact plants because enzymes
  and substrates spatially separated (i.e.
  glycoside in epidermis and enzymes in mesophyll)
• Only when cells crushed are they brought together

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Effects of HCN

• HCN inhibits ETS
• HCN completely inhibits the transfer of electron
  to oxygen by cytochrome oxidase final step in
  the ETS
• This can lead to cell death and possibly even the
  death of the organism if the concentration is
  high enough

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Electron Transport System
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HCN- Glycoside variability

• Amounts of cyanogenic glycosides highly variable
  in any one species
• Control of variability both genetic and
  environmental
• Cassava (Manioc esculenta) major food crop with
  cyanogenic glycosides
• Rosaceae (Rose Family) contains cyanogenic
  glycoside - amygdalin

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Cassava glycosides

• Traditional preparation removes toxins usually
• In Africa reported cases of cassava paralysis due
  to HCN poisoning the motor region of brain

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Cassava
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Amygdalin

• Found in seeds, stems, leaves and roots of
  apricots, cherries, peaches, apples, almonds -
  bitter almond
• Acute poisoning has occurred when young children
  have eaten apricot pits
• Laetrile - controversial cancer treatment

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Laetrile

• Apricot pits are rich in amygdalin
• Ground up for preparation of laetrile, a
  controversial cancer treatment
• Theoretically, laetrile releases HCN only in the
  presence of tumor cells and selectively destroys
  them
• Not proven and laetrile not approved for cancer
  therapy in the United States

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Amygdalin
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Glucosinolates

• Sulfur and nitrogen containing cmpds in the
  mustard family
• Provide flavor of cabbages, broccoli,etc
• Feeding repellants and insect toxins
• Provide the potency of mustard and horseradish
  antibiotic properties of these
• Rape seed development