METABOLISM 149 



of feed ingredients is brought about largely or wholly by their 

 agency and is often effected by different enzyms in successive 

 stages. Thus the ptyalin of the saliva converts starch into 

 maltose while the further conversion of the latter into dextrose 

 is effected by the maltase of the intestine. Quite similar are 

 the successive actions of pepsin, trypsin and erepsin on the pro- 

 teins. In all these cases, as well as in the even more familiar case 

 of the diastase of germinating seeds, the enzyms act at a dis- 

 tance from the cells which produce them and have, therefore, 

 been called extracellular enzyms. 



209. Intracellular enzyms. From the fact that the most 

 obvious cases of enzym action were those in which the ferment 

 acted at a distance from the cells producing it, enzyms came to 

 be regarded as substances whose action belonged in a different 

 category from that of living cells. A sharp distinction was 

 drawn between unorganized substances, acting substantially 

 as chemical reagents, and organisms producing chemical changes 

 by virtue of their life. The action of the yeast plant upon sugar 

 afforded a typical example of this distinction. It was shown 

 that yeast secreted an enzym (invertase) which was capable of 

 inverting sucrose independently of the action of the yeast cell, 

 while, on the other hand, the alcoholic fermentation of mono- 

 saccharids was held to be a vital function of the living yeast 

 cells. 



Buchner, however, in 1897, showed that by suitable means 

 there could be extracted from yeast a substance (zymase) 

 which fermented the simple sugars exactly like yeast in the 

 absence of any living organism whatever; i.e., it acted as an 

 enzym. It became evident, then, that the yeast cell ferments 

 monosaccharids not because it is alive but because it contains 

 zymase. The only essential difference between the yeast fer- 

 mentation and that, for example, produced by diastase or by 

 the invertase of yeast is that the enzym normally acts within 

 the cell which produces it. Later it was shown that what is 

 true of the yeast fermentation is true also of the fermentation 

 caused by the lactic acid bacillus. It, too, is due to an intra- 

 cellular enzym which can be separated from the cell and act 

 independently. Investigators are inclined, therefore, to re- 

 gard all fermentation as the work of enzyms, some of which, 

 like the digestive enzyms, are excreted by the cells and may 



