60 PHYSICAL CHEMISTRY IN BIOLOGY. 



acid and this then into alcohol and carbon dioxide. In the same 

 manner the decomposition of protein into amino acids, with proteoses, 

 peptones, arid 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 (zymogens, 

 proenzymes). The best-studied example of a zymogen is trypsinogen, 

 which is contained in the pancreatic juice and converted into an active 

 enzyme by the enterokinase of the intestine. As this kinase is destroyed 

 by heat it also seems to be an enzyme-like body. (See Chapter IX.) 

 Trypsinogen can also be activated by lime salts. In certain other cases 

 the presence of bodies which resist temperature and are dialyzable 

 and therefore not enzymes, are necessary besides the real organic enzyme. 

 Thus the presence of an acid is necessary for the action of pepsin. R. 

 MAGNUS 1 has been able to separate by dialysis from a solution of liver- 

 lipase a body which is necessary for the action upon amyl salicylate. 

 Enzymes made inactive by dialysis can be activated again by the addi- 

 tion of boiled enzyme or the concentrated dialysate. HARDEN and 

 YOUNG 2 on filtering yeast-press juice through earthenware filters impreg- 

 nated 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 tempera- 

 ture, is added. According to BUCHNER and KLATTE 3 the constituent 

 of the zymase which is resistant to temperature is destroyed by lipase. 

 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 

 EuLER. 4 Their action is probably different in different cases, and 

 differs also from the activating action of the kinases. 



Laws of Action of the Enzymes. The enzyme reactions always 

 take place in heterogeneous media, where on one hand the enzyme exists 

 as colloid and on the other the substrate in many cases is a colloid (starch 



1 Zeitschr. f . physiol. Chem., 42, 149, 1904. 



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

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



3 Bioch. Zeitschr., 8, 520, 1908. 



* Zeitschr. f. physiol. Chem., 57, 92, 1906. 



