CHAPTER IX 



GLUCOSIDES AND GLUCOSIDE-SPLITTINO ENZYMES 



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

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

 these groups replaced by the CgHnOg — 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. Sometimes more than one monosaccharide takes part in the 

 composition of the glucoside. (These various relationships are shown in 

 the accompanying 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 significance 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 



