Drugs Containing CHAPTER Carbohydrates and Derived Products 14 14.1. INTRODUCTION importance is the fact that sugars unites with a wide variety of other compounds to form glycosides and secondary Carbohydrates, as the name suggest, were defined as a metabolites. Mucilage, as found in marshmallow root and group of compounds composed of carbon, hydrogen and psyllium seeds, act as water-retaining vehicles, where as oxygen in which the latter two elements are in the same gums and mucilage, which are similar in composition and proportion as in water and were expressed by a formula properties, are formed in the plant by injury or stress and (CH O) , that is, hydrates of carbon. 2 n usually appear as solidified exudates; both are typically The term ‘carbohydrates’ arose from the mistaken belief that substances of this kind were hydrates of carbon, composed of uronic acid and sugar units. The cell walls because the molecular formula of many substances could of the brown seaweeds and the middle lamellae of higher be expressed in the form C (H O) , for example, glucose plant tissues contain polysaccharides consisting almost X 2 Y entirely of uronic acid components. (C6 H12 O6), sucrose (C12 H22 O11), etc. In these examples, the hydrogen and oxygen are present in the same ratio Low molecular weight carbohydrates are crystalline, as in water. But this definition has certain drawbacks as soluble in water and sweet in taste, for example, glucose, given below: fructose, sucrose, etc. The high molecular weight carbo- It should be kept in mind that all organic compounds hydrates (polymers) are amorphous, tasteless and relatively containing hydrogen and oxygen in the proportion less soluble in water, for example, starch, cellulose, inulin, found in water are not carbohydrates. For example, etc. formaldehyde HCHO for the present purpose written 14.2. CLASSIFICATION as C(H2O); acetic acid CH3COOH written as C3(H2O)2; and lactic acid CH3CHOHCOOH written as C3(H2O)3 are not carbohydrates. Carbohydrates Also, a large number of carbohydrates such as rhamnose (C6H12O5), cymarose (C7H14O4), digitoxose (C6H12O4), etc., are known which do not contain the usual propor- tions of hydrogen to oxygen. Simple sugar (Saccharides) Polysaccharides Finally, certain carbohydrates are also known which contain nitrogen or sulphur in addition to carbon, hydro- gen and oxygen. From the above discussion, it can be concluded that Monosaccharides Disaccharides Trisaccharides Tetrasaccharides the definitions described above are not correct; however, carbohydrates are now defined chemically as polyhydroxy aldehyde or polyhydroxy ketones or compound that on Monosaccharides hydrolyses produce either of the above. The term ‘monosaccharides’ is employed for such sugars Carbohydrates are among the first products to arise as a that on hydrolysis yield no further, lower sugars. The result of photosynthesis. They constitute a large proportion general formula of monosaccharides is Cn H2n On. The of the plant biomass and are responsible, as cellulose, for monosaccharides are subdivided as bioses, trioses, tetroses, the rigid cellular framework and, as starch, for providing pentoses, hexoses, heptoses, depending upon the number an important food reserve. Of special pharmacognostical of carbon atoms they possess. Chapter-14.indd 159 10/12/2009 5:08:30 PM 160 TEXTBOOK OF PHARMACOGNOSY AND PHYTOCHEMISTRY Bioses and are rarely found accumulated in plants, for example, They contain two carbon atoms. They do not occur free glucoheptose and manoheptose. in nature. Disaccharides Trioses They contain three carbon atoms, but in the form of phos- Carbohydrates, which upon hydrolysis yield two molecules phoric esters, for example, glyceraldehydes. of monosaccharides, are called as disaccharides. Tetroses Sucrose Hydrolysis Glucose + fructose (sugarcane) They contain four carbon atoms, for example, erythrose, threose, etc. Maltose Hydrolysis Glucose + Glucose (malt sugar) H O H O Lactose Hydrolysis Glucose + Galactose (cow’s milk) H O C C C H C OH HO C H H C OH H C OH H C OH Trisaccharides CH OH 2 CH OH CH OH D -(+)-glyceraldehyde 2 2 As the name indicates, these liberate three molecules of D -(-)-erythrose D -(-)-threose monosaccharides on hydrolysis. Pentoses Raffinose Hydrolysis Glucose + fructose + galactose (in beet) (sugarcane) They are very common in plants and are the products of Gentianose Hydrolysis Glucose + Glucose + fructose hydrolysis of polysaccharides like hamicelluloses, mucilages (gentian roots) and gums, for example, ribose, arabinose and xylose. Tetrasaccharides H O H O H O C C C Stachyose, a tetrasaccharide, yields on hydrolysis, four H C OH HO C H H C OH molecules of monosaccharide, found in manna. H C OH H C OH HO C H H C OH H C OH H C OH Polysaccharides CH OH CH2 OH CH2 OH 2 On hydrolysis they give an indefinite number of mono- D -(-)-ribose D -(-)-arabinose D -(+)-xylose saccharides. By condensation, with the elimination of water, polysaccharides are produced from monosaccharides. Hexoses Depending upon the type of product of hydrolysis these They are monosaccharides containing six carbon atoms and are further classified as Pentosans and Hexosans. Xylan are abundantly available carbohydrates of plant kingdom. is pentosan, whereas starch, insulin and cellulose are the They are further divided into two types: aldoses and ketoses. examples of hexosans. β They may be obtained by hydrolysis of polysaccharides like Cellulose is composed of glucose units joined by -1, 4 starch, insulin, etc. linkages, whereas starch contains glucose units connected with α- 1, 4 and α- 1, 6 units. Polyuronides, gums and Aldoses : Glucose, mannose, galactose mucilages are the other pharmaceutically important poly- Ketoses : Fructose and sorbose saccharide derivatives. H O H O H O OH OH C C C CH OH 2 CH 2 OH CH2 OH O O HO O O HO O H C OH HO C H H C OH C O HO O O HO O O HO H CH OH HO CH OH HO C H HO C H 2 H 2 HO C OH HO C H OH OH CH OH CH OH H C OH H C OH HO C H H C OH O 2 O HO O 2 O HO O HO O O HO O O H C OH H C OH H C OH H C OH HO HO H CH2 OH H CH2 OH CH OH CH OH CH OH CH2 OH 2 2 2 D -(+)-glucose D -(+)-mannose D -(+)-galactose D -(-)-fructose Cellulose β-1, 4 linkages Heptoses 14.3. TESTS FOR CARBOHYDRATES They contain seven carbon atoms, vitally important in the The following are some of the more useful tests for sugars photosynthesis of plant and glucose metabolism of animals and other carbohydrates. Chapter-14.indd 160 10/12/2009 5:08:30 PM DRUGS CONTAINING CARBOHYDRATES AND DERIVED PRODUCTS 161 Reduction of Fehling’s Solution test tube. The bottom of the test tube is heated for 30–60s. A pink or red stain appears on the reagent paper. To the solution of carbohydrate, equal quantity of Fehling’s solutions A and B is added. After heating, brick red pre- 14.4. BIOSYNTHESIS OF cipitate is obtained. CARBOHYDRATES Molisch Test Production of Monosaccharides by The test is positive with soluble as well as insoluble car- bohydrates. It consists of treating the compounds with Photosynthesis α-naphthol and concentrated sulphuric acid which gives Carbohydrates are products of photosynthesis, a biologic purple colour. With a soluble carbohydrate this appears process that converts electromagnetic energy into chemical as a ring if the sulphuric acid is gently poured in to form energy. In the green plant, photosynthesis consists of two a layer below the aqueous solution. With an insoluble, classes of reactions. One class comprises the so-called light carbohydrate such as cotton wool (cellulose), the colour reactions that actually convert electromagnetic energy into will not appear until the acid layer is shaken to bring it in chemical potential. The other class consists of the enzymatic contact with the material. reactions that utilize the en ergy from the light reactions to fix carbon dioxide into sugar. These are referred to as the dark Osazone Formation reactions. The results of both of these types of reactions are most simply summarized in the following equation: Osazones are sugar derivatives formed by heating a sugar chlorophyll solution with phenylhydrazine hydrochloride, sodium 2H O + CO + light (CH O) + H O + O acetate and acetic acid. If the yellow crystals which form 2 2 2 2 2 are examined under the microscope they are sufficiently Although this equation summarizes the overall rela- characteristic for certain sugars to be identified. It should tionships of the reactants and products, it gives no clue be noted that glucose and fructose form the same osazone as to the nature of the chemical intermediates involved (glucosazone, m.p. 205°C). Before melting points are taken, in the process. The elucidation of the reactions by which osazones should be purified by recrystalization from alcohol. carbon dioxide is accepted into an organic compound and Sucrose does not form an osazone, but under the condi- ultimately into sugars with regeneration of the carbon tions of the above test sufficient hydrolysis takes place for dioxide acceptor was a major achievement in biosynthetic the production of glucosazone. research. The pathway of carbon in photosynthesis, as worked out primarily by Calvin and coworkers, is pre- Resorcinol Test for Ketones (Selivanoff’s Test) sented in Figure 14.1. CO A crystal of resorcinol is added to the solution and warmed 2 on a water bath with an equal volume of concentrated Metabolic pool hydrochloric acid. A rose colour is produced if a ketone is Photosynthesis present (e.g. fructose, honey or hydrolysed inulin). Pyruvate ATP Test for Pentoses Glucose Glucose-6-phosphate Heat a solution of the substance in a test tube with an equal volume of hydrochloric acid containing a little phloroglu- Monosaccharides Hexose-6-phosphate cinol.