CHAPTER X 



GLUCOSIDES 



Attention has been drawn (Chapters v and viii) to the fact that in the 

 plant, compounds containing hydroxyl groups often have one or more of 

 these groups replaced by the CeHnOg — residue of glucose. Such com- 

 pounds are termed glucosides. The substances in which this substitution 

 most frequently occurs are of the aromatic class, and the glucosides may 

 be regarded, on the whole, as ester-like compounds of carbohydrates with 

 aromatic substances. The non-sugar portion of the glucoside may vary 

 widely in nature, and may be, for instance, an alcohol, aldehyde, acid, 

 phenol, flavone, etc. The sugar constituent is most frequently glucose, 

 but pentosides, galactosides, mannosides and fructosides are also known. 

 Sometime^ more than one monosaccharide takes part in the composition 

 of the glucoside. (These various relationships are shown in the accom- 

 panying table.) The inclusion of all glucosides in a class is in a sense 

 artificial: the character held in common (with very few exceptions) is 

 that, on boiling with dilute acids, or, by the action of enzymes, hydrolysis 

 takes place, and the glucoside is split up into glucose (or other sugar) 

 and another organic constituent. A number of compounds occurring as 

 glucosides have already been dealt with, for example, the tannins and 

 flavone, flavonol and anthocyan pigments, but, in these cases, the sig- 

 nificance of the compounds lies rather in the nature of their non-sugar 

 constituents than in the fact of their being glucosides. 



There are, however, a number of glucosides which have been grouped 

 together and are more readily classified in this way than in any other. 

 Some of them, doubtless, have come into prominence as glucosides on 

 account of their association with well-known and specific enzymes, as, 

 for instance, the glucoside amygdalin associated with the enzyme emul- 

 sin, and the glucoside sinigrin with the enzyme myrosin. 



The hydrolyzing enzymes are by no means always specific, for in 

 vitro one particular enzyme may be able to hydrolyze several glucosides. 

 Many glucoside- splitting enzymes have been described, though there is 

 no reason to suppose that each glucoside is only acted upon by an 

 enzyme specific to that glucoside. It is likely moreover that some of 

 the different enzymes described will probably prove to be identical. 



In some cases where more than one monosaccharide is attached to 



