Glycosides Part 4 PDF

Preview

Summary

Part four of Glycosides:Thioglycosides, Cardiac glycosides...

Description

PHM 4453 11 (FOP) GLYCOSIDES 4 (Thioglycosides, Cardiac glycosides) 7.0. ISOTHIOCYANATE GLYCOSIDES 7.1 Introduction Also called glucosinolate compounds The aglycone part of the compounds contains sulphur and nitrogen. Compounds abundant in Brassicaceae, Capparidaceae and Recedaceae. They occur sporadically in the Euphorbiaceae, Caricaceae, Moringaceae. Hydrolysis of these yield volatile genins of thiocyanate structure e.g., mustard oils. Best known compounds sinigrin and sinalbin glycosides, occur in black mustard and white mustard seeds respectively. The glycosides and their specific enzymes are found in different cells in the seeds. By the destruction of the cell walls, the cpds are exposed to the hydrolyzing enzymes.

The general structure of thioglycosides:

The anion is called the glucosinolate ion, ( R ) may be aliphatic or aromatic. The cation (X) may be a simple metal ion, e.g K+ in sinigrin; or a complex organic cation, e.g., sinapine ion of sinalbin. Sinigrin gives upon hydrolysis, glucose, allylisothiocyanate (volatile oil of mustard) and potassium acid sulphate. Glycoside sinalbin gives a phenolic isothiocyanate, (acrinyl isothiocyanate), glucose and the acid sulphate of a quaternary alkaloid, sinapine. Glucosinolates are water soluble compounds as a result of their ionized sulfate and hydrophilic thioglucose moieties, hence, not easily separated & purified.

Structure of thioglycosides: cont’d

S-GL S-GL CH2

HO

CH-CH2 -C N-OSO3 K

Sinigrin

CH2

C N-OSO3 - Sinapine+

Sinalbin

7.. Mustard seedBot source: Black or brown mustard is the dried ripe seed of Brassica nigra or of B junceaCruciferae/Brassicaceae. ConstituentsBlack mustard seeds contain sinigrin and myrosin also contain 35-40% FO White mustard seed –Sinapsis alba contain the glucoside of sinalbin and myrosin. Uses Black and white mustard seeds are used externally in the form of plasters as rubefacients and counter irritants. Recently, these glucosinolates have been suggested to have many activities e.g: anti cancer (Cruciferous Complex –Neo-Life) anti-inflammatory, anti-oxidant anti- bacterial, anti-fungal. (molecules-21-00416.pdf) Test for Sulphur containing glycosides They give black precipitate of silver sulphate upon treatment with AgNO3 solution.

8.0. CARDIOACTIVE DRUGS 8.1 Introduction Are C23 or C24 steroidal glycosides which exert on the failing heart, a slowing and strengthening effect (positive ionotropic effect) Cardiac glycosides are an important class of naturally occurring constituents whose actions include both beneficial and toxic effects on the heart. Cardiac glycosides are composed of two structural features: the sugar (glycone) and the non-sugar (aglycone - steroid) moieties. Cardiac glycosides possess a steroid nucleus with: an unsaturated lactone at the C-17 position, and at least one glycosidic residue at the C-3 position, via β-linkage. The sugar may be unusual digitoxose, L-rhamnose, thevetose common glucose, etc.

Cardiac steroids are widely used in the treatment of congestive heart failure (CHF) and for treatment of atrial fibrillation and flutter. Their toxicity remains a serious problem. CGs are abundant in the plant families Apocynaceae and Asclepiadaceae. Also found in some Liliaceae (e.g Convallaria), Moraceae, Brassicaceae, Stercuraceae, Euphorbiaceae, Introduction cont’d Tiliaceae, Celastraceae, Leguminosae, Scrophularaceae. Some toad skins also contain such. Two types of aglycone are known and may be distinguished according to whether there is a five- or six-membered unsaturated lactone ring at C-17 position of the steroid portion. The types are known respectively as cardenolides and bufanolides or bufadienolides. Cardenolide & bufanolide steroidal skeletons

NomenclatureThe substituent group at the 17-position defines the class of cardiac glycoside. The cardenolides have an unsaturated butyrolactone ring while the bufadienolides have an α-pyrone. The cardiac glycosides occur mainly in plants from which their names have been derived. Digitalis purpurea, Digitalis lanata, Strophanthus gratus, and Strophanthus kombe are the major sources. The word digitoxin refers to an agent derived from a Digitalis spp., consisting of digitoxigenin (aglycone) and sugar moieties (three). Sugar moietyOne to 4 sugars are present in most CGs attached at the 3β-OH group. The sugars most commonly involved include L-rhamnose, D-glucose, D-digitoxose, Ddigitalose, D-diginose, D-sarmentose, L-vallarose, and D-fructose, etc. These sugars predominantly exist in the cardiac glycosides in the β-D-conformation-some exceptions; L-rhamnose, L-vallose, L-thevetose, etc.

The sugar moiety is important only for the partitioning and kinetics of action. The presence of acetyl group on the sugar affects the lipophilic character and the kinetics of the entire glycoside. NomenclatureNature has synthesized various cardiac glycosides with differing sugar skeletons but based on relatively few aglycone structures! 8.2. THE CARDENOLIDES Cardenolides are more common than the bufanolides and are particularly abundant in the Apocynaceae, Asclepiadaceae and Scrophularaceae, Liliaceae. The cardenolides consist of: the digitalis, apocynaceae and other groups such as convallaria. 8.2.1.DIGITALIS GLYCOSIDES 8.2. 1.1. Digitalis Leaf Bot source: Digitalis (Purple Foxglove Leaves) consists of dried leaves of Digitalis purpurea, family Scrophulariaceae, dried at a temperature below 600C, immediately after collecting the leaves. Leaves are then packed in airtight containers, protected from light. The leaves should not contain more than 5 % of moisture. It is required to contain not less than 0.3% calculated as digitoxin. Geog source; Holland & Eastern Europe. Digitalis grows well in in sandy soil, with a trace of element manganese. Collection: It is generally agreed that 1st year leaves collected July-Aug have the highest yield. It is also believed that glycoside content peaks in the early afternoon. Purp glyc B & glucogitaloxin peak at 40%. P g A is formed last & becomes the major component at 50% 0f total glycs in the mixture. Collection cont’d/constituents . Leaves contain 0.2-0.45% of both primary & sec glycs. The 3 primary CGs contain at C-3 of the aglycone a linear chain of three digitoxose sugar moieties terminated by glucose. Purp glycosides A & B constitute the principal active constituents of the fresh leaves. On drying, enzyme hydrolysis takes place with the loss of terminal glucose to give digitoxin, gitaloxin & gitoxin, respectively. Gitaloxigenin series, with a formyl grp at C-16 are less stable than digitoxin & gitoxin series. Digitoxin and gitoxin are therefore the main active components of the dried drug. Poor storage will lead to further hydrolysis and complete loss of activity. Other glycosides present in small proportions involve the same aglycones & contain digitalose and glucose-they exist in mono- & diglycosides. In this series is verodoxin which is said to be 3* as toxic as gitaloxin.

D purpurea also contains anthraquinones. Saponins e.g. digitonin, and flavonoids e.g luteolin- have also been isolated from the leaves

Structures of the common aglycones O

O

R1 12 13

11 1 2

14 8

3 H

O

5 4

R2

16

9 10

17

OH

15

7 6

Substitution pattern in some cardenolides CARDENOLIDE R2 Substituent R1 (C-16) Substituent (C-12) Digitoxigenin

H

H

(series A)

Gitoxigenin

H

OH

(series B)

Gitaloxigenin

H

OCHO ( series E)

Digoxigenin

OH

H

(series c)

Sugar substituents at C-3 position COULD BE: DX = Digitoxose DX (AC) =Acetyldigitoxose G = Glucose. Other sugars in CGs: L-rhamnose, L-thevetose, L-oleandrose, D-fucose, D-digitalose, D-sarmentose, D-cymarose, D-glucomethylose.

Cardiac glycosides derived

Uses D purpurea preps have a tonic effect on the failing heart., slowing it, but STRENGTHENING & REGULATING IT without requiring more oxygen. Stimulates urine flow, hence lowering the blood volume & the load on the heart. Has also been used in internal haemorrhage & other diseases. Has a cumulative effect, so medical supervision is necessary. 8.2.1.2. Digitalis lanata leaf – Bot source: Consists of the dried leaves of D lanata, fam Scrophulariaceae, is also important. Geog source: Indigenous to central & s-e Europe. Cultivated in Holland, Equador & USA. Constituents;10 glycs of D lanata resemble those of D purpurea but are acetylated at the digitoxose moiety (e.g lanatoside C) next to the terminal glucose. This confers crystalline characters on the cpds, making them easier to isolate. Partial hydrolysis occurs during drying & storage giving products the same as in D purpurea. De-acetylation will also produce products the same as in D purpurea. In addition to the above series, two others involving digoxigenin & diginatigenin are found in the leaves. Lanatoside C & other important med glycs attain their highest levels in the 2nd year plants. Anthraquinones similar to those in D purpurea have been recorded.

Uses Isolation of lanatosides & digoxin. Digoxin has become the most widely used drug in the Rx of CHF. Digoxin acts similarly to Digitalis Leaf but is more rapidly absorbed from the GIT. Hence digoxin is valuable for rapid digitalization in the Rx of atrial fibrillation and CHF. Lanatoside C is less well absorbed than digitoxin but is less cumulative. 8 .2. 2. APOCYNACEAE GROUP.

Over 12 genera of this family are known to contain cardioactive glycosides i.Strophanthus glycosides: Three glycosides-- K-strophanthoside (a trioside), K-strophanthin-B (bioside) and cymarin (a monoside) have been isolated from dried ripe seeds of different Strophanthus species. They are all based on the aglycone strophanthidin. Strophanthus glycosides O

11 1 2

CHO 9 10 5

3 O

4

12 13 14 8

OH 7

OH 6

O

17

K- strophanthidin 16 15

Cymarin K- strophanthin B K- strophanthoside

Cymarose B-glucose a-glucose

The seeds of Strophanthus gratus contain another glycoside named Ouabain or (Gstrophanthin), which yield on hydrolysis rhamnose and the aglycone ouabagenin. Ouabagenin differs from K-strophanthidin in having 2 additional (OH) groups at C-1 and C-11 and having a 10 alcoholic group at C-10 instead of the aldehydic group. Ouabain is also the principle glycoside of the wood of the African Acokanthera schimperi – formerly A ouabaio.

Ouabain (G-strophanthin),

O

O

OH OH OH CH2 1

2

11 9

10

O

5 4

17 13 16 14

8

3 Rhamnose

12

OH

15

7 6

OH

Uses K- strophanthin is cardiotonic. Glycoside has similar actions to digitalis with less cumulative effects. Adminstered parenterally because it is poorly absorbed via oral route. Ouabain can be isolated in pure crystalline form, and has been used as a reference standard in biological assays of cardiac glycosides. Ouabain is twice as active as K-strophanthin, hence given IV in acute cardiac failure. Onset of action is within 5-10 mins, reaching peak effect in half-2 hours, and sustaining effect for 1-3 days. Arrow poison in E Africa. ii. Oleander ii-Nerium oleander, An evergreen ornamental, flowering plant of Mediterranean origin, contains oleandrin and digitalinum verum, gitoxigenin, digitoxigenin, etc, in the leaves. PoisoningOleander ingestion-accidental by children and suicide attempts- causes many cases of poisoning world-wide.

iii. Thervetia peruviana glycosides Iii-Thevetia peruviana; The yellow oleander- is a rich source of the glycoside thevetin A. Thevetin is considered useful in cases of mild myocardial insufficiency and where digitalis intolerance exists. Thevetin is used mainly in continental Europe. Thevetin A-has 2 glucose & thevetose. On loss of the 2 glucose mols, peruvoside is formedalso cardioative.

Peruvoside consists of L-thevetose linked to aglycone cannogenin.

8.2.3. Miscellaneous CGs sources e.g. Convallaria –the aerial parts of the lily of the valley, Convallaria majalis, fam Liliaceae, found in Europe. Principal glycoside is convallotoxin which on hydrolysis gives strophanthidin & (-)-rhamnose. Many other glycosides have been identified. Used in herbal medicine for its cardioactive properties which are similar to those of digitalis but less cumulative.

8.3.BUFANOLIDES Bufads are found in plant families-Liliaceae, Iridaceae, Melanthaceae, Rununcularaceae, Santalaceae and in animals-Bufonidae, Ciluridae & Lampyridae. This group of cardioactive agents includes the squill glycosides (the scillarens) and the toad poison, bufatoxin. The genins of squill glycosides differ from those of the cardenolides in two important aspects: a)They have six membered doubly unsaturated lactone ring in position C-17. b) They have at least one double bond in the steroid nucleus. The most important is squill 8.3. i.Squill-white squill Bot source: Squill BP consists of the dried sliced bulbs of Drimia maritima- formerly Urginea maritima- family Liliaceae collected after the plant has flowered and from which the membranous outer scales have been removed. Constituents: Scillaren A is the most important constituent of squill. Others include scillaren B, glucoscillaren A+ others. The bulb also contains flavonoid derivs of quercetin, kaempferol, fructan sinistrin, lignan, mucilage. Uses. Therapeutically, bufadienolides not used as cardioactive drugs because of their low therapeutic index. However, squill is important as an expectorant and has been widely used in the treatment of cough. Scillarens are poorly absorbed from the GIT, they have a short duration of action, and are not cumulative. In small doses, squill provokes mild gastric irritation causing a reflex secretion from the bronchioles- hence its application in the treatment of cough. In larger doses, it causes vomiting.

8.3.ii Indian squill Bot source: Consists of the dried , usually longitudinally sliced, bulb of Drimia indica. Uses: Small doses are expectorant, otherwise in larger doses have digitalis-like action.. Chemical identification of cardiac glycosides Cardiac glycosides (CG) are steroidal in nature, hence give +ve with Liebermann’s test. CG that contain deoxy-sugars are distinguished by Keller Kiliani’s test. Liebermann–Burchard test-or acetic anhydride test Used for the detection of steroids & triterpenes. Since this test uses acetic anhydride and sulfuric acid as reagents, caution must be exercised so as not to receive severe burns. Formation of first red then a green or green-blue color after a few minutes is positive. The colour is due to the hydroxyl group (-OH) steroid reacting with the reagents and increasing the conjugation of the un-saturation in the adjacent fused ring. Method Drug to be tested is dissolved in dry chloroform in a dry test tube. Several drops of acetic anhydride and then 2 drops of conc.H2SO4 are added and mixed carefully. After the reaction finished, the concentration of cholesterol /steroid can be measured using spectrophotometry. Keller Kiliani’s testThese sugars are found in Digitalis and Strophanthus, Nerium, Thevetia, etc, spp. Method Powdered plant material –say 1-2g- is boiled with 70% alcohol for 2-3 min then filtered. To 5 ml of filtrate,10ml of water, 0.5ml of strong solution of lead sub-acetate is added and the mixture shaken and filtered. The filtrate is shaken with 5ml of chloroform then allowed to separate. The chloroform layer is removed by pipetting off and gentle evaporation in a porcelain dish. The cooled residue is dissolved in 3ml of glacial acetic acid containing two drops of 5% ferric chloride soln. The mixture is transferred to the surface of 2ml.conc H2SO4. At the junction of the liquids, a reddish brown layer is produced at the junction of the two liquids, and the upper layer slowly becomes bluish-green, darkening with standing. Cardenolides are distinguished from the scillarens as follows ;  Kedde’s reagent; 3,5-dinitrobenzoic acid –red-violet color is produced. This complex is assayed at λmax540 nm.  Baljet’s reagent; alkaline sodium picrate-red-orange color λmax 495 nm.,  Legal’s test; alkaline solution of sodium nitroprusside-red color λmax 470nm.. These compounds react with the active methylene of the five membered lactone ring (in alkaline medium) to give characteristic colors. Summary/SARs The aglycones of all cardiac glycosides that act as cardioactive agents are steroidal in nature,

Have specific action on cardiac muscle, increasing tone, excitability and contractility of this muscle thus allowing the failing weakened heart to function more efficiently. β-OH at position C-3, which is always involved in a glycosidic linkage to a mono, di, tri, or tetra- saccharide. The presence of another β-OH group at C-14. Unsaturated 5 or 6- membered lactone ring at position C-17, also in the β configuration. The A/B ring junction is usually (cis), while the B/C ring junction is always (trans) and the C/D ring junction is in all cases (cis). Additional OH groups may be present at C-5, C-11 and C-16. Cardiac glycosides that have α-β unsaturated 5-membered lactose ring in position C-17 are known as cardenolides. These are represented by the digitalis and apocynaceae group. Digitalis glycosides contain methyl group at C-10, while strophanthus glycosides are characterized by presence of either an aldehydic (CHO) or primary alcoholic (CH2OH) group at C-10. Cardiac agents that have doubly unsaturated 6-membered lactone ring in position C-17 are referred to as bufadienolides. This group includes the squill glycosides and the toad venom, bufotoxin. The glycone portion at position C-3 of cardiac glycosides may contain four monosaccharide molecules linked in series. Thus, from a single genin/aglycone, one may have a monoside, a bioside, a trioside or a tetroside. With the exception of D-glucose and L-rhamnose, all the other sugars that are found in cardiac glycosides are uncommon deoxy-sugars e.g., Digitoxose, Cymarose, Thevetose, etc Isolation difficulties: Major difficulty in the isolation of 10 glycosides from the crude drug because 1ry glycosides are converted into secondary glycosides by hydrolyzing enzymes. Other difficulty is the existence of several closely related glycosides in the same drug, which are extremely difficult to separate and purify.

END...