480 LECTURE XX. 



yeast maltase, and to the production of isolactose from glucose and galac- 

 tose by kephir-lactase, and to the synthesis of amygdalin from mandelo- 

 nitrile glucoside and grape-sugar with the aid of yeast maltase. 1 From these 

 observations there can be no doubt that fermentations are reversible pro- 

 cesses. This does not prove, however, that the conditions in the tissues and 

 cells are such that the known ferments are active in this direction to any 

 great extent. It is very tempting to refer all metabolic processes to fer- 

 mentation. At one place there is a decomposition, at another, construction, 

 according to the requirements of the cells. The ferment acts as an inter- 

 mediary. Many enigmas would thus be solved at one stroke, and many 

 apparently different processes referred to one simple basis. It is certainly 

 possible that such a significance really belongs to the ferments, and that 

 they dominate the entire metabolism. We must, however, confine our- 

 selves to the facts, and build on them alone as foundation. We are, in the 

 first place, impressed with the fact that all the fermentation syntheses so 

 far carried out satisfactorily do not give rise to products to which the fer- 

 ment is accustomed, with the possible exception of the amygdalin synthesis. 

 We do not obtain maltose from grape-sugar, but an isomer, isomaltose; 

 nor do we obtain lactose from glucose and galactose, but only isolactose. 

 These facts are certainly not without significance. Can the maltase again 

 decompose isomaltose, or kephir-lactase hydrolyze isolactose? These 

 were unsolved problems until recently. Thanks are due E. F. Armstrong 2 

 for thoroughly studying these fermentation syntheses, and especially 

 the formation of maltose and isomaltose. Armstrong started with the 

 fact which we have not previously mentioned, that d-glucose can exist in 

 stereo-isomeric forms. If we crystallize grape-sugar from alcohol, we only 

 obtain the a-form. If this is kept for several days at 105 degrees, it goes 

 over into the /? form. The two modifications have different optical 

 behaviors. It has been attempted to represent this type of stereo-isomer- 

 ism by means of formulae, although a satisfactory explanation is still lack- 

 ing. If glucose is dissolved in methyl alcohol, containing hydrochloric 

 acid, glucosides corresponding to both forms are produced. As we have 

 already seen, only one of these varieties is split by maltase, the other only 

 by emulsin. The variety hydrolyzed by maltase is the a form. If we 

 apply these observations to maltose and isomaltose, we can regard the 

 former as glucose-a-glucoside, and the latter as glucose-/?-glucoside. In 

 this case also, the maltase is only capable of splitting the a form, while 

 the /?-glucoside remains unattacked. Now maltase produces synthetically 

 /?-glucoside, i.e., the glucoside which it cannot decompose. The following 

 experiments completely cleared up these relations. If glucose was treated 



1 Cf. Lecture III, pp. 37, 38. 



8 Pro. Roy. Soc. 76 (B), 592 (1905); 19, 209 (1903); Jour. Chem. Soc. 83, 1305 

 (1903). 



