72 CARBOHYDRA TESMONOSACCHARIDES 



Normal urine contains minute traces of glucose, and sometimes 

 larger amounts, especially after a meal which is very rich in carbo- 

 hydrates. Such glycosuria is known as Alimentary Glycosuria and is 

 devoid of significance unless it occurs too frequently and readily, in 

 which case it may possibly indicate disturbance of the functions of the 

 pituitary gland. In certain pathological conditions or under experi- 

 mental conditions much profound and serious glycosurias may occur. 

 These will fall under consideration in a later chapter. 



Galactose is found in important quantities in two places in the 

 animal kingdom, namely combined with glucose to form milk-sugar 

 or Lactose; and in the form of glucoside-like compounds, the Cere- 

 brosides, which are found in the brain. 



THE LACTONE-STRUCTURE OF SUGARS. 



Before proceeding to the consideration of the disaccharides, it is 

 important to review some recent accessions to our knowledge of the 

 sugars which have led us to reconsider in some degree the structural 

 formulae by means of which we have hitherto represented them. It is 

 necessary to enter thus deeply into the subject of the configuration of 

 the sugar molecule because a clear understanding of these questions 

 has already fundamentally contributed to our knowledge of the mode 

 of action of ferments, and is unquestionably destined to do so even to a 

 greater degree than heretofore. ' In considering the enzymatic hydrol- 

 ysis and synthesis of the disaccharides we shall have occasion to refer 

 very frequently to the facts which are about to be described. 



It will be recollected that in compounds containing only four 

 asymmetric carbon atoms, such as we have been assuming the hexoses 

 to be, only sixteen stereo-isomers are possible. Now, as a matter of 

 fact, it has long been known to sugar-chemists that the optical rotatory 

 power of solutions of d-glucose is not a constant quantity. The 

 optical rotatory power of fresh solutions changes gradually, sometimes 

 increasing, but more usually falling, until a constant value is ultimately 

 reached. This constant value is the same for all glucose solutions 

 which have attained equilibrium, but the initial rotatory power of 

 fresh solution may be as much as twice as great as the final constant 

 rotatory power. This phenomenon is variously known as Mutaro- 

 tation, Multirotation and Birotation. 



Analogous phenomena in other solutions are generally attributed 

 to the presence of two or more different, optically active substances, 

 of different rotatory power and convertible into one another. Adopting 

 this point of view, Emil Fischer first suggested, in explanation of the 

 phenomenon of mutarotation, that the glucose undergoes hydration 

 in solution, with the formation of an alcohol of lower rotatory power, 

 thus: 



