Secondary Plant Metabolites Active Constituents

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Secondary Plant Metabolites(Active Constituents)
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• The secondary metabolites act as a protective
  agents.
• They constitute as the waste product of
  metabolism (detoxification).
• They are biosynthesized to aid the producers'
  survival.
• Thus they are energy producer and
  physiologically active.
• Secondary metabolites are divided into several
  classes of various organic compounds.

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I- Carbohydrates and related compounds

• Compounds composed of carbon, hydrogen and oxygen
  as polyhydroxy aldehydes or ketone alcohols
  sucrose, lactose, starch, gums, mucilage and
  pectin.
• 1- Gums
• They are translucent, amorphous substances that
  are frequently produced in higher plants as a
  protective after injury.
• Gums are heterogeneous in composition. Upon
  hydrolysis arabinose, galactose, glucose, mannose
  and various uronic acids are produced.
• The uronic acids may form salts with calcium,
  magnesium and other cations. Examples of gums are
  gum Acacia and gum Tragacanth.

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• 2- Mucilages
• 1- They are polysaccharide complexes formed from
  sugar and uronic acid units.
• 2- They are insoluble in alcohol but forming a
  viscous non-adhesive colloidal solution with
  water.
• 3- They are stained red with Ruthenium Red
  reagent (pectose type present in Senna, buchu),
  and with solution of Corallin soda (callose type
  found in Squill). Others are stained by methylene
  blue (neutral type of Fenugreek).

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• 3- Pectin
• It is formed of partially methoxylated
  polygalactouronic acids.
• Pectin is a coarse or fine powder, yellowish
  white in colour, almost odourless and has a
  mucilaginous taste.
• Soluble in 20 parts of water and the solution is
  viscous, colloidal and acidic to litmus.
• One part of pectin heated in nine parts of water
  forms a stiff gel.
• Pectin is used as a suspending agent and is an
  ingredient in many antidiarrheal formulations.

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II- Glycosides

• Definition
• Glycosides are non-reducing organic compounds
  that yield on hydrolysis with acids, alkalis or
  enzymes
• 1- A sugar (or glycone, formed of one or more
  sugar units).
• 2- A non-sugar moiety (also called aglycone or
  genin).
• Glycosides are widely distributed in nature. They
  usually accompanied with enzymes used in either
  their synthesis or hydrolysis.

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• Glycosides are considered as acetals or sugar
  ethers.
• Glycosides are generally soluble in water and
  hydroalcoholic solvents due to the hydrophilic
  nature of the sugar.
• The higher the sugar content the less soluble in
  organic solvent. The aglycones are soluble in
  organic solvents.

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Classification of glycosides A- Phenolic
glycosides

• They produce phenolic aglycone when subjected to
  hydrolytic processes. The phenolic aglycone may
  be
• 1- Simple phenolic glycosides e.g. Arbutin
  present in Uva ursi leaf

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• 2- Flavonoid glycosides
• They occur in plant in both free aglycone and /or
  as glycosides.
• This group is regarded as C6-C3-C6 compounds in
  which each C6 moiety is a benzene ring e.g.
  Diosmin glycoside of Buchu and hespiridin
  glycoside of Citrus species.
• Chemical test flavonoides KOH ? canary yellow
  colour

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• 3- Anthraquinone glycosides
• They are pharmacologically active constituents of
  several laxatives and purgatives of plant origin
  e.g. Senna, Aloe, Cascara, Frangula, Rhubarb.

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• 1- Borntrager's test for anthraquinone
  glycosides
• Boil the powder with dil. HCl, filter, cool,
  shake with organic solvent, separate organic
  layer, shake with NH4OH, the aqueous layer
  becomes rose pink or cherry red.
• 2- Modified Borntrager's test in case of
  dianthrone e.g. sennosoides
• a- Boil the powder with alc. KOH, filter.
• b- Add dil. HCl to the filtrate and extract with
  ether.
• c- Oxidize with H2O2.
• d- Add NH4OH to the ethereal extract and shake,
  a rose red colour is produced in the aqueous
  layer.

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• 4- Anthocyanin glycosides
• they are structurally related to flavonoids and
  coloured.
• They constitute the pigments of the coloured
  petals of the flowers.
• Their colours are affected by the pH of the cell
  sap and ranges from red in acidic medium to blue
  in alkaline medium. e.g. Red Rose petals,
  karkadeh and corn silk.

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B- Cyanophore glycosides (Cyanogenic)

• They yield HCN as one of the products of
  hydrolysis.
• Examples are amygdalin in bitter almond,
  prunasin in wild cherry and linamarin in linseed.
• Chemical test by Guignard test turns the yellow
  colour of sodium picrate paper into orange.

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C- Thioglycosides (thiocyanate or sulphated
glycosides)

• S-glycosides, S atom is present conjugated to
  glucose and a second S atom present in a
  sulfonated oxime grouping.
• Singrin from black mustard seeds giving allyl
  isothiocyanate (mustard irritant gas) upon
  hydrolysis.

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D- Steroidal cardioactive glycosides

• These drugs are used in medicine as mainly to
  increase the tone and contractility of the
  cardiac muscle.
• Also as diuretic due to the increased renal
  circulation.
• Thet are present in leaves of Digitalis, seeds of
  Strophanthus and the bulb of Squill.

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Structure activity relationship (SAR)

• Steroidal nucleus with alcoholic OH group at C3
  at which sugar is attached.
• Attachment of unsaturated lactone ring at C17.
  the lactone ring is present either as a 5-
  membered ring (Cardinolides) e.g. lannatosides in
  Digitalis or as 6- membered ring (bufadienolides)
  e.g. Scillarin in Squill.
• Deoxy sugar, e.g. digitoxose.

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Detection of cardiac glycosides

• Several chemical tests could be used for
  identification of cardiac glycosides
• Colour reactions due to aglycone moiety
• A- Reactions due to (-CH2-) group of the lactone
  ring
• These are characteristic for the 5-mambered
  lactone ring of cardenolides
• 1- Kedde's test card. Kedde's reagent
  (3,5-dinitrobenzoic acid NaOH ? violet colour.
• 2- Baljet's test card. Baljet's reagent (picric
  acid NaOH) ? orange or red.

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• B- Reactions due to the steroidal nucleus
• Liebermann's test card. in glacial acetic acid
  H2SO4 ? red, violet, blue to green.
• Reactions due to the presence of 2-deoxy sugar
  unit in the sugar moiety
• Keller-Killiani's test card. in glacial acetic
  acid containing traces of FeCl3, conc. H2SO4 on
  the wall of the test tube ? blue ring is formed
  in between the two layers.

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E- Saponins

• e.g. saponins of Quillaia bark
• This group of glycosides is characterized by the
  following
• it foams in aqueous solution forming froth.
• It causes haemolysis of red blood cells.
• Drugs containing saponins are usually
  sternutatory and irritating the mucous membrane
  of the eyes and the nose.
• They are highly toxic when injected into blood
  stream.
• They are used in cleaning industrial equipments
  and fabrics, also as a powerful emulsifier of
  certain resins, fats and fixed oils.

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III- Tannins

• They are non-crystalline compounds that with
  water form colloidal solutions possessing an acid
  reaction and a sharp astringent taste.
• They precipitate gelatin and alkaloids.
• They precipitate protein and can combine with
  protein of animal hide (Gold beater's test),
  rendering them resistant to proteolytic enzymes
  and so prevent their putrefaction.

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• Uses of drugs containing tannins
• Astringent in GIT and skin (haemostatic).
• In treatment of burns.
• Tanning of leather.
• Antidote of alkaloidal and heavy metals
  poisoning.
• Preparation of paints and ink.
• Types of true tannins
• True tannins have molecular weight 1000-5000.
  there are two main types of true tannins.
• Pseudotannins
• compounds of lower molecular weight than true
  tannins, they don't respond to the gold beater's
  skin test.

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IV- Alkaloids

• Alkaloids, which mean alkali-like, are basic
  nitrogenous compounds of biological origin, they
  are physiologically active.
• They occur in the form of salts with organic
  acids, or in combination with specific acids e.g.
  opium alkaloids occur with meconic acid .
• generally alkaloidal bases are insoluble in
  water but soluble in organic solvents. While
  alkaloidal salts are soluble in water sparingly
  soluble in organic solvents.

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• Their physiological actions vary widely being
  Analgesic and Narcotic e.g. morphine and codeine
• Central stimulant e.g. caffeine and strychnine
• Mydriatic e.g. atropine
• Myotic e.g. pilocarpine
• Antiasthmatic e.g. ephedrine
• Antihypertensive e.g. reserpine
• smooth muscle relaxant e.g. atropine and
  papaverine
• skeletal muscle relaxants e.g. d-tubocurarine.

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Detection of alkaloids

• Alkaloidal precipitants
• Most alkaloids are precipitated from neutral or
  acidic solution by a number of reagents which
  contain certain heavy metals e.g. Mayer's reagent
  (potassium mercuric iodide solution) gives creamy
  white ppt. with most alkaloids except caffeine.
• Alkaloidal colour reagents
• The most common Dragendorff's reagent gives
  orange colour.

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V- Volatile oils

• Volatile or essential oils are volatile in steam.
• They are secreted in oil cells, in secretion
  canals or cavities or in glandular hairs.
• They are generally mixtures of hydrocarbons and
  oxygenated compounds derived from these
  hydrocarbons.
• These oxygenated constituents mainly determine
  the odour and taste of volatile oils.

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• Volatile oils are used for flavoring e.g. oil of
  Lemon, in perfumery e.g. oil of Rose
• as spices e.g. Pepper, Clove , Cardamom
• and for their therapeutic action as antiseptic
  e.g. Thyme, antispasmodic e.g. Melissa, Menthe,
  and carminative e.g. Chamomile.

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VI- Resins, Gum-resins and similar substances

• Resins are more or less solid, amorphous
  substances of complex chemical nature, on heating
  they soften and finally melt. They are insoluble
  in water but dissolve in alcohol, chloroform and
  ether.
• Resins are associated with volatile oil, so
  called oleoresin.
• With gum and called gum resin.
• With oil and gum and called oleo-gum resin.
• Resins give brown colour with iodine reagent.
• VII- Others
• Enzymes, vitamins, hormones, antibiotics, natural
  insecticides, etc.........