52 GENERAL AND PHYSICO-CHEMICAL. 



salicylate. Enzymes made inactive by dialysis can be activated again 

 by the addition of boiled enzyme or the concentrated dialysate. HARDEN 

 and YOUNG 1 on filtering yeast-press juice through earthenware filters 

 impregnated with gelatin, have found different constituents of the zymase 

 on the filter and in the filtrate. The true enzyme remains on the filter. 

 This alone is inactive, but becomes active when the other part which 

 has passed through the filter, and which is dialyzable and resistant to 

 temperature, is added. This part is consumed during fermentation 

 and therefore the enzyme becomes inactive. After the addition of, best, 

 boiled press-juice to this the fermentation begins again (see also Chapter 

 III). Certain of the just-mentioned substances which are resistant 

 to heat, whose presence are necessary for the action of certain enzymes, 

 are ordinarily called co-enzymes. As they are not to be classified with 

 the enzymes, they are more correctly called activators, as suggested by 

 EuLEE. 2 Their action is probably different in different cases, and 

 differs also from the activating action of the kinases. 



Many enzymes are secreted by the cells as such or as proenzymes. 

 They act outside of the cells in which they were formed, or they act after 

 having been transformed into the enzyme, and hence are called secre- 

 tion enzymes or extracellular enzymes. Besides these extracellular 

 enzymes we also have another group which acts within the cells, hence 

 are intracellular and therefore are called intracellular enzymes or endo- 

 enzymes. To this group belongs, beside the yeast zymase, numerous 

 enzymes such as oxidases and hydrolytic enzymes. 



Formation and Secretion of Enzymes. The investigations of PAWLOW 3 

 and his pupils upon the formation and secretion of the enzymes active 

 in the alimentary tract are very important. According to these investiga- 

 tions the amount of secretion of the glands and the behavior of the enzymes 

 contained in the secretion are dependent upon the amount and com- 

 position of the food taken and in such a manner that the kinds and 

 amounts of enzymes are appropriate for the digestion of the food- 

 stuffs (see Chapter VIII). Similar results were also obtained by WEIN- 

 LAND 4 who found that the pancreas does not normally contain any 

 lactase but did contain this enzyme after feeding the animal with 

 milk or milk sugar. This has been substantiated by BAiNBRiDGE. 5 

 Analogous experiments have been made with salivary ptyalin by NEIL- 



. Physiol. Soc., 32 (1904); Proc. Chem. Soc., 21, 189 (1905); Proc. Roy. 

 Soc., 77 (ser. B), 405 (1906); ibid., 78, 369 (1906). 

 2 Zeitschr. f. physiol. Chem., 57, 92 (1908). 



3 Arbeit der Verdauimgsdrusen, Wiesbaden, 1898, s. 51. 



4 Zeitschr. f. Biol., 38, 607 (1899); 40, 386 (1900). 

 6 Journ. of Physiol., 31, 98 (1904). 



