ENZYMES. 51 



Like the colloids the enzymes only diffuse very slowly and the dif- 

 fusion through membranes does not occur in most cases; only certain 

 membranes such as collodion tubes allow certain enzymes to pass 

 through. The collodion tubes can be impregnated in such a way with 

 lecithin or cholesterin that the diffusion is very slight. The same 

 applies to the filtration through collodion membranes (BIERRY and 

 ScHAEFFER). 1 It must not be forgotten in such experiments that the 

 membrane can adsorb a considerable part of the enzyme (BECHHOLD). 2 



Just as it is difficult to prepare an enzyme free from non-enzymotic 

 contaminations, so also is it difficult to exclude the possibility that a 

 so-called enzyme is not a mixture of several related enzymes. In fact 

 the several enzymotic processes proceed step by step, and it is possible 

 that the various steps are caused by different enzymes. Thus the 

 decomposition of protein into amino-acids, with proteoses, peptones, 

 and polypeptides as intermediary products, may be the result of the 

 activity of several enzymes which are active one after another or are 

 parallel with one another in activity. Erepsin does not attack genuine 

 proteins, but completes the decomposition which has been begun by 

 other enzymes (pepsin, trypsin). 



The enzymes are formed within the living cells. In certain cases 

 the cells do not secrete the complete enzyme, but substances which are 

 transformed first outside of the cells into active enzymes. These pre- 

 liminary steps or mother substances of the enzymes have been called 

 proenzymes or zymogens. These under certain conditions are changed 

 into enzymes and in certain cases this is brought about by the inter- 

 action of special but not well known substances which have been called 

 kinases (see Chapters V and VIII). In other cases the transformation 

 of the zymogen into the active enzyme is brought about by well defined 

 chemical substances. Thus the proenzymes of pepsin and of rennin are 

 activated by acids (see below on the retardation of enzyme action and 

 also Chapter VIII). 



In certain other cases the presence of bodies which resist temperature 

 and are dialyzable and therefore not enzymes, are necessary or helpful 

 besides the real organic enzyme. Thus the presence of an acid is neces- 

 sary for the action of pepsin and hydrocyanic acid, according to MENDEL 

 and BLOOD, 3 favors to a high degree the action of papain (a plant pro- 

 tease). R. MAGNUS 4 has been able to separate by dialysis, from a solu- 

 tion of liver-lipase, a body which is necessary for the action upon amyl 



1 Compt. rend. soc. biol., 62, 723 (1907). 



2 Zeitschr. f. physik. Chem., 60, 257 (1907). 

 s Journ. of biol. Chem., 8, 177 (1910). 



4 Zeitschr. f. physiol. Chem., 42, 149 (1904). 



