240 GLUCOSIDES 



treatment with this salt, the excess of lead being subsequently 

 removed by hydrogen sulphide. 



Aqueous solutions of glucosides frequently have a bitter 

 taste and are Isevo-rotatory ; they do not reduce Fehling's 

 solution until liberation by hydrolysis of the monosaccharide 

 which, as has been stated, may be a hexose, pentose, or methyl 

 pentose or a mixture of two of more of these. In the plant 

 hydrolysis is effected by an appropriate enzyme which may be 

 specific, or may be capable of splitting several glucosides. 

 In the process of extraction, precautions must accordingly 

 be taken to prevent interaction between the enzyme and its 

 substrate ; this is best effected by treating the material 

 with boiling alcohol, in order to destroy the enzyme, prior to 

 the extraction of the glucoside with alcohol or water. 



Bourquelot's method of investigating plants for glucosides 

 has been extensively employed ; it depends on the fact that 

 all glucosides which are hydrolysed by emulsin are laevo- 

 rotatory, but after hydrolysis become dextro-rotatory and 

 acquire reducing properties. 



The glucoside and the enzyme may in some cases be 

 contained in the same cell and only come into contact with 

 each other on injury, or during certain phases in the plant's 

 metabolism. 



On the other hand, the enzyme and substrate may be 

 secreted in distinct tissues ; an example of this is furnished 

 by the seeds of Lunaria biennis in which the cotyledons 

 secrete the enzyme whilst the integument co^itains the gluco- 

 side. If the seeds are skinned and the cotyledons and testas 

 are separately ground, no smell of mustard oil is produced ; 

 but if the two are ground together, the myrosin acting upon 

 the sinigrin contained in the seed-leaves, liberates allyliso- 

 thiocyanate. 



THE CONSTITUTION OF THE GLUCOSIDES. 



The constitution of the natural glucosides can be best 

 understood by a brief consideration of the simplest known 

 artificial glucosides which have been synthesized from glucose. 



The lactone formula for glucose with its asymmetric 



