Carbohydrates Pharmacognosy I Mosul University College of Pharmacy

Carbohydrates Pharmacognosy I Mosul University/ College of Pharmacy L. A. Dilbreen Barzanji

Carbohydrates • As the names implies, carbohydrates consist of carbon, hydrogen and oxygen with the last two elements usually present in the same proportions as in water. • They constitute a large proportion of the plant biomass and are responsible, as cellulose, for the rigid cellular framework and, as starch, for providing an important food reserve. Of special pharmacognostical importance is the fact that sugars unite with a wide variety of other compounds to form glycosides. • Mucilages, as found in marshmallow root and psyllium seeds, act as water-retaining vehicles, whereas gums, which are similar in composition and properties, are formed in the plant by injury or stress and usually appear as solidified exudates.

Sugars (saccharides) A- Monosaccharides • These sugars contain from 3 -9 carbon atoms, but those with 5 and 6 carbon atoms (pentoses and hexoses) are accumulated in plants in greatest quantity. • Glucose has an aldehyde group and is therefore called an aldose or ‘aldo’ sugar; fructose has a ketone group and therefore called a ketose. • Uronic acids are produced by oxidation of the terminal groups to –COOH (e. g. glucuronic acid from glucose and galacturonic acid from galactose).

B- di-, tri- and tetra-saccharides • these sugars may also be called bioses, trioses and tetroses. They are theoretically derived from two, three or four monosaccharide molecules, respectively. • One of the commonest plant disaccharides is sucrose. • Hydrolysis is brought about by suitable enzymes or by boiling with dilute acid.

C- Polysaccharides • Polysaccharides are derived from monosaccharides in an exactly similar manner to the formation of di-, tri- and tetrasaccharides. • The name ‘oligosaccharide’ is often applied to saccharides containing from 2 -10 units. • In polysaccharides the number of sugar units is much larger and the number forming the molecule is often only approximately known. • The hydrolysis of polysaccharides, by enzymes or reagents, often result in succession of cleavages, but the final products is hexoses or pentoses or their derivatives. The term ‘polysaccharide’ may usefully be taken to include polysaccharide complexes which yield in addition to monosaccharides their sulphate esters, uronic acids or amino sugars.

Starches • Starch constitutes the principle form of carbohydrate reserve in the green plant and is to be found in especially in seeds and underground organs. • A number of starches are recognized for pharmaceutical use. They include maize, rice, wheat and potato starches. • Preparation of starches: many patented processes are in use for particular starches, and the procedure adopted depends on the degree of purity desired and the nature of compounds from which the starch has to be freed. • For example; Cereal starches have to be freed from cell debris, oil, soluble protein matter and the abundant insoluble proteins (gluten). Potato starch in other hand, is associated with vegetable tissue, mineral salts and soluble proteins.

• Wheat and similar starches were at one time prepared by kneading the ground material in a stream of water, the gluten remaining as a sticky mass, while the starch separated on standing from the milky washings. • Macroscopical characters: starch occurs in irregular, angular masses or as a white powder. It is soluble in cold water but form a colloidal solution on boiling with about 15 times its weight of water, the solution forming a translucent jelly on cooling. • Uses: starch finds extensive use in dusting powders, in which its absorbent properties are important. In mucilage form it used as a skin emollient, as a basis for some enemas and as a antidote in the treatment of iodine poisoning. Starch is also used as a tablet disintegrant.

Fructans • Fructans are D-fructose polymers each chain being terminated by a single D-glucosyl residue. They are found in nature as oligosaccharides with up to 10 units and as polysaccharides with up to 50 units. • The best-known fructan, and the most important pharmaceutically is inulin.

Inulin • It derives its name from the dahlia, Inula helenium, from which it was first isolated in the 19 th century. • It is sparingly soluble in cold water but readily dissolves at around 70 0 C without gelatinizing. Inulin is not metabolized by the body and is excreted unchanged. As inulin injection it is used for the measurement of glomerular filtration rate.

Algal gelling agent Alginic acid • Large quantities of brown seaweeds are collected from many of the colder waters of the world. • The North Atlantic rockweeds (e. g. Ascophyllum nodosum) are cut either by hand with stickles or by means of various designs of floating ‘combine harvesters’. • After collection, the raw, wet seaweed may either go immediately for processing or be fuel- or sun-dried to 12 -17% moisture content, in which form it has an indefinite storage life. • Alginic acid, a hard, horny polysaccharide. It is composed of residues of D-mannuronic and L-guluronic acids; the chain length is long and varies with the method of isolation and the source of algae.

• The alginates, particularly the sodium salt (because of their greater chemical reactivity) have certain advantages over agar, starch, pectin, vegetable gums and gelatin. • Alginates find applications as stabilizing, thickening, emulsifying, gelling and film-forming agents in pharmaceutical industries (formulation of creams, ointments, pastes, jellies and tablets). • Alginic acid is also used in tablet and liquid preparations for the control of gastrooesophageal reflux.

Gums and mucilages • Gums and mucilages have similar constituents and on hydrolysis yields a mixture of sugars and uronic acids. • Gums are considered to be pathological products formed upon injury of the plant, conversely, mucilages are generally normal products of metabolism formed within the cell and may represent storage material, a water-storage reservoir or a protection for germinating seeds. They are often found in quantity in the epidermal cells of leaves, e. g. senna, in seed coats (linseed, psyliium etc. ), roots (marshmallow) and barks (slippery elm).

Tragacanth • Tragacanth is the air-hardened gummy exudates, flowing naturally or obtained by incision, from the trunk and branches of Astragalus gummifer. • The gum is white or very pale yellowish-white in colour, translucent and horny. It breaks with a short fracture, is odourless and has little taste. • Tragacanth swells into a gelatinous mass when placed in water, but only a small portion dissolves. • Like other gums, tragacanth is composed of sugar and uronic acid units. • Uses: tragacanth is used in pharmacy as a suspending agent for insoluble powders, etc. or as a binding agent in pills and tablets.

Stercula gum • Sterculia (Karaya gum) is the dried gummy exudates obtained from the tree Sterculia urens (f. Sterculiaceae). • Characters: good-quality gum occurs in irregular almost colorless, translucent, striated masses weighing up to 25 g or more. • Karaya gum has a marked odour of acetic acid. • In water sterculia has low solubility but swells to many times its original volume.

• Constituents: partial acid hydrolysis of sterculia yields D-galactose, L-rhamnose, D-galacturonic acid, aldobiouronic acids, an acid trisaccharides and acetic acids. • Uses: the glanular grades are used as bulk laxative, being second only to psyllium seed in use in this respect. • The powdered gum is used in lozenges, pastes and denture fixative powders.

Acacia gum • Acacia (gum Arabic) is a dried gum obtained from the stem and branches of Acacia senegal. • Some gum exudes from trees as a result of cracking of the bark, but the most esteemed. • Constituents: The glycan composition of the gum is variable, depending on source. Thus that derived from A. Senegal contains a high proportion of D-galactose relative to L-arabinose, whereas the reverse holds for the gum obtained from A. seyal. • Uses: as a general stabilizer in emulsions and as a pharmaceutical necessity in lozenges, etc. its demulcent properties are employed in various cough, diarrhea and throat preparations but it is incompatible with readily oxidized materials such as phenols, and the vitamin A of cod-liver oil.

Psyllium • The dried, ripe seeds of Plantago psyllium is used in medicine. • Constituents: all the seeds contain mucilage in the epidermis of the testa. The seeds may be evaluated by measuring the volume of mucilage produced after shaking the seeds with water and allowing to stand. • Two fractions have been separated from the mucilage; one is soluble in cold water, and the other in hot water giving a highly viscous solution which gels on cooling. • On hydrolysis fractions yield D-xylose, L-arabinose & aldobiouronic acid. • Uses: plantago seeds are used as demulcent and in the treatment of chronic constipation.

Honey • Honey is a saccharine substance deposited by the hive bee, Apis mellifera and other species of Apis, in the cells of honeycomb. • The best honey is that derived from flowers such as clover and heather, obtained from hives that have never swarmed, and separated from the cut comb either by draining or by means of centrifuge. • Honey obtained by expression is liable to contaminated with the wax. • The nectar of certain flowers (e. g. of species of Eucalyptus ) may give the honey a somewhat unpleasant odour and taste; nectar fromsome plants like Datura stramonium is known to give poisonous honey.

• Preparation of honey may involve melting at a moderate temperature, skimming off any impurities that collect on standing and, if necessary, adjusting the water content using refractive index measurement. • Characters : honey, when freshly prepared, is a clear, syrup liquid of a pale yellow or reddish-brown colour. On keeping, it tends to crystallize and become opaque and granular. The odour and taste depend very largely on the flowers used in its preparation.

• Constituents: honey consist mainly of invert sugar and water. It contains small quantities of sucrose, dextrin, formic acid, volatile oil, wax and pollen grains. • The most likely adulterants are artificial invert sugar, sucrose and commercial liquid glucose. • The tests of purity of the BP/EP purified honey should be noted. • Uses: honey is chiefly used in pharmacy as a component of linctuses and cough mixtures.

Quiz