CARBOHYDRATES 19 



ALCOHOLS (Glycitols) 



Reduction of the carbonyl group of a sugar yields a polyhydroxy alcohoL A few of 

 these are well-known natural products. Except for lack of the reducing function, they 

 generally resemble the sugars in their properties. It must be noted that reduction of the 

 anomeric carbon changes the possibilities for isomerism, so that the same sugar-alcohol 

 may be derived from several different sugars, as sorbitol from glucose, gulose, or 

 fructose. 



Glycerol is undoubtedly the best -known sugar -alcohol, but it is important as a 

 building block of the lipids rather than in its free form. The higher sugar -alcohols are 

 given the ending -itol. The four carbon erythritol occurs in algae, lichens, and grasses. 

 Ribitol is found free in Adonis vernalis but more importantly as a part of the ubiquitous 

 riboflavin molecule. The six and seven carbon sugar -alcohols are the most common 

 representatives of this class. Mannitol and sorbitol are found in many plants, the former 

 in exudates, the latter especially in fruits but also in leaves. Only two natural heptitols 

 are known. An octitol has been reported in avocado (6). 



The carbocyclic inositols, although not derived from sugars by simple reduction, 

 are conveniently treated here since they are quite similar in chemical properties. The 

 particular isomer mjo-inositol (formerly called meso-inositol) occurs widely in plants 

 both free and as phytic acid, an anhydride of its hexaphosphate. Phytin is a calcium - 

 magnesium salt of phytic acid. Certain lipids are also derived from inositol, and the 

 sugar beet contains galactinol, 1-O-a-D-galactosyl >».vo-inositol. In addition to myo- 

 inositol, pinitol (a methyl ether of D-inositol), and quebrachitol, (a methyl ether of L- 

 inositol) are very widespread. Plouvier (7) has found inositol methyl ethers in a wide 

 variety of plant species. Structures and occurrence of these and other inositol compounds 

 are given in Figure 2-6. A review of the chemistry and natural occurrence of inositols 

 and related compounds has been presented by Angyal and Anderson (8). Mj'o- inositol acts 

 as an essential growth factor for certain plant tissue cultures (9). 



SUGAR ACIDS 



Oxidation of one or both of the terminal carbon atoms of a sugar molecule to a car- 

 boxyl group yields a sugar acid. Oxidation of the aldehyde group forms an aldonic acid. 

 If the other terminal carbon is oxidized, the product is a uronic acid. The simplest of 

 these would be glyoxylic and glycolic acids; but since they are important in organic acid 

 metabolism rather than carbohydrate metabolism, they are treated in Chapter 3. D- 

 glyceric acid is chiefly important as its phosphate esters, which are intermediates in 

 carbohydrate breakdown and in photosynthesis. The tartaric acids may be thought of as 

 erythrose derivatives with both terminal carbons oxidized to carboxyl groups. The six- 

 carbon sugar acids occur in small amounts, chiefly as intermediates in the degradation 

 of hexoses to form pentoses and as building blocks and degradation products of pectin, 

 gums, and mucilages. All are water soluble and frequently exist as lactones so that 

 their acidic properties are not apparent on titration in the cold. The uronic acids are 

 easily decarboxylated by boiling with acid. This leads to errors when polysaccharides 

 containing them are broken down by acidic hydrolysis. Structures of the important sugar 

 acids appear in Figure 2-9 (p. 33). 



The most important free sugar acid, L-ascorbic acid, is not only a vitamin for man 

 but may play a role in the metabolism of plants. One established function is as a coenzyme 

 for a mustard oil glycosidase (10). It is a strong reducing agent and may be readily dis- 

 tinguished by this property, although the four -carbon enediol, dihydroxy-funiaric acid 

 occurs in nature to a limited extent and has similar properties. Ascorbic acid owes its 



