ACTION OF THE ENZYMES. 157 



It has been shown by KJELDAHL that sucrase acts best in a 

 slightly acid medium. A greater concentration of the acid, 

 be it inorganic or organic, acts deleteriously, as does also an 

 alkali in any concentration. In fact the activity of sucrase 

 is markedly influenced by a concentration of alkali, the pres- 

 ence of which cannot even be recognized by our ordinary 

 indicators. As the intestinal juice is neutral or if anything 

 slightly acid, we see that sucrase acts, so far as reaction is 

 concerned, under favorable conditions in the body. 



13. Lactase is a ferment which has the power of acting 

 upon the disaccharide lactose (milk-sugar) and converting it 

 into the monosaccharides d-glucose (dextrose) and d-galac- 

 tose. 



Lactose Water Dextrose Galactose 



Various yeasts usually serve as a source of the ferment which 

 has been studied more particularly by BEYERiNCK, 1 EMIL 

 FisCHER, 2 ARMSTRONG, 3 and WEiNLAND. 4 EMIL FISCHER ob- 

 tained the ferment most readily by making an aqueous ex- 

 tract of Saccharomyces Kefir and precipitating the ferment 

 with alcohol. The reversible activity of this enzyme has 

 been demonstrated by EMIL FISCHER and ARMSTRONG, who 

 found that milk-sugar (or at least a disaccharide closely re- 

 lated to it) appears in a concentrated mixture of d-glucose 

 and d-galactose under the influence of lactase, when the 

 whole is kept in a thermostat at 35 C. 



The fact that milk-sugar is one of the commonest constitu- 

 ' nts of our food, especially in suckling animals, gives the 

 occurrence of lactase in certain organs its physiological im- 

 portance. The exceedingly contradictory statements of dif- 



1 BEYERINCK: Centralbl. f. Bacteriol., 1889, VI, p. 44. 



2 EMIL FISCHER: Berichte d. deut. chem. Gesellsch., 1894, XXVII, 

 3481. 



3 EMIL FISCHER and ARMSTRONG: Berichte d. deut. chem. Gesellsch. . 

 1902, XXXV, p. 3144. 



4 WEINLAND: Zeitschr. f. Biol., 1899, XXXVIII, p. 606. 



