Plant steroids and glycosides

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Plant steroids
Anna Drew
with grateful acknowledgement for inspirational teaching received at
The School of Pharmacy, University of London
PLANT STEROIDS
• Used for:
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–
–
–
–
–
replacement therapy (male +female)
athletes (glucocorticoids)
skin conditions (hydrocortisone)
antifertility pill (oestrogens + progesterones)
cancer (breast, testes, prostrate)
rheumatoid arthritis
• Industrial demand may be met by plant sources
or replaced by synthetic sources (expensive)
Structure
• Hydrocarbons
– (3 x 6C) + (1 x 5C) = tetracyclic triterpenoid type
– ring junctions sometimes contain 3y methyl groups
– normally side chains are at C17 (classified by this)
– and functional groups at C3 (-O or -OH groups)
– also at C11 (-O, –OH gives oxygen function)
– shape very important for biological activity
– 3D determined by ring junction AB – CD
Trans AB
isomer
Cis AB
isomer
• AB-CD trans junction tend to have a flat, planar
structure
– important for hormonal activity
• AB-CD cis junction are bent or buckled
– allows them to fit on heart / smooth muscle and blood
protein receptor sites
– poisonous steroids – some used in heart disease
Hormones – plant sources
• GROUP 1 sapogenins
• GROUP 2 phytosterols
• occur as glycosides
linked to a sugar
• polar, soluble in alcohol
and alcohol/water
mixtures
• occur in leaves -> roots,
rhizomes
• occur as ester linked to
fatty acids
• non-polar, soluble in
hexane and petroleum
spirits
• occur in fruits and seeds
Occur in very large amounts in plants
– 10-25% by weight of plant material
Sapogenins
• Sapogenin = steroid nucleus
• Saponin = glycosides + sugars
– ‘soap-like’ in nature
– have been used to poison fish
• accumulates in gills preventing O2 transfer
– also frogs and toads
• breathe through skin and hence are killed
• not poisonous to mammals when eaten
– not absorbed in intestine or stomach
– may irritate bowel causing diarrhoea
– few effects
• if injected different
– used in arrow poisons
– cause haemolysis of red blood cells
• breaks down red blood cell membrane
• haemoglobinuria
• some used as emulsifying agents
• interested in the aglycone from a saponin
• Saponins occur widely in plants
– some economically important ones:
Sources:
[1] Dioscoreaceae (yam family)
• Dioscorea genus – dicots – vines
– sweet yam – food source, very low steroid content
– bitter yam – Mexico, South America – high content
[2] Liliaceae family
• monocots – Far East, Phillipines
– Smilax or Yucca
• very important since these provide sapogenins for manufacture of
corticosteroids
[3] Amaryllidaceae
• Agave sisalana sisal leaf, East Africa
[4] Solanaceae
• can be used when supply of [1] and [2] short or too expensive
• Solanum sp. contain steroidal saponins
– as well as tropane alkaloids, atropine, etc
– eg tomato, potato, woody nightshade
[5] Scrophulariaceae
• Digitalis seeds full of steroids, rich source
[6] Leguminosae
• Trigonella-foeum-graecum fengreek seed
Structure:
– based on steroid nucleus
• flat trans- shape
– right shape steroid to make hormones
– occurs in a high concentration in plants
– spiroketal side chain easily oxidised off (leaves unstable
progesterone)
• spiroketal side chain at C17
• 2 isomers at C25 due to free rotation around it
– no other isomers occur naturally
25 α
iso-series
25 β
neo-series
• sugars attach at C3 to make sapogenins saponins
– tend to have quite large molecular weight
» eg 3-12 sugars = polysaccharide side chain
– common sugars: xylose, galactose, rhamnose, glucose
– combination of these sugars is usually a branched
complex structure with high mol wt (ie not linear sugar
chains)
– lipid soluble steroid part + water soluble sugar part
» can orientate at water|oil or air|water interface
Tigogenin
Structure courtesy of
www.chemblink.com
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–
–
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simplest sapogenin
has correct configuration from which to make steroids
occurs with the isomer neotigogenin
widely distributed in plants:
• yam, digitalis seeds, fenugreek seeds
Diosgenin
Structure courtesy of
www.chemblink.com
– can obtain prednenolone and progesterone from it
– occurs with isomer yamogenin
– occurs with some tigogenin in
• fenugreek seeds and Mexican wild yam Dioscorea mexicana
(and Japanese types)
– hard to cultivate yams – tubers underground – may
take years to grow large enough – mostly taken from
wild
Hecogenin
Structure courtesy of
www.chemblink.com
– from the sisal plant, various species of yucca
• Philipines and Far East
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isomer is sisalgenin
keto function at C12 important
corticosteroids have C11 =O group giving activity
here C11 cannot be substituted; C12 =O enables
halogenation at C11, then =O removed at C12
Commercial extraction
• sources crushed
– tubers – yams; seeds – fenugreek, digitalis; leaves –
sisal
• fermentation
– add excess water in fermenting vat and leave 24-48
hour
– saponins are covalently bonded into cellulose wall
– own enzymes act on the polysaccharides in the cell
wall to liberate them
• filtration to collect plant powder
• acid hydrolysis to split off saponins from sapogenins
– equal HCL, MeOH, H2O
• plant material dried in an oven
• Soxhlet extraction with petroleum spirit
– to distill over saponins
– crystallise out in receiver
– 10g/100g yam tuber – high yield – economic
• recrystallise
– using various solvents depending on desired
compound
– can be carried out on a large scale
– cheap
• no chromatographic process
• materials cheap (H2O, HCl) – petroleum spirit can be
recovered
• recrystallisation expensive but gives a high yield
• p’ceutical companies will buy compounds in pure
Analysis of plant material
• Qualitative:
– TLC on sulphuric acid to indicate spot position
(chloroform solvent)
• Quantitative:
– i) colorimetric assay – sulphuric acid produces orange
colour with steroids
– ii) IR spectrometry – 960cm-1
– need a lot of plant material
– iii) GLC micromethod – draw up assay with suitable
standard and do many samples in one day
– quickest
– qualitative and quantitative
Commerical use
• production of steroids from diosgenin
• before 1940 isolate from animal glands or urine
– expensive
• 1940 Marker (USA) discovered a process –
essentially same process is still used
• diosgenin extracted from Mexican yam
• then spiroketone chain is opened up....
• in theory process gave 100% yield
• progesterone known to prevent ovulation – tried to
produce ‘the pill’ (1950)
• now have combination pills
• 1950-1960 corticosteroids needed
– antiinflammatory, anticancer, antirheumatoid
• hydrocortisone and cortisone (which can be fluorinated)
couldn’t be produced from progesterone
[1] Fermentation
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arose by accident when making antibiotics from Rhizopus
needed steroids in medium to grow
produced 11 keto progesterone
analysed fermentation to confirm pregnenolone / progesterone
were producing 11 keto progesterone (from which hydrocortisone
can be made)
– biotechnology expensive
[2] Hecogenins
– more economical
11 keto
progesterone
hydrocortisone
Marker
11 keto
diosgenin
tautomers
– NB Diosgenin can be used to produce hydrocortisone but a
fermentation stage is needed to introduce O- at C11 keto α
position of pregnane nucleus