416 CHEMICAL DYNAMICS 



somewhat with the nature of the protein or protein salt employed 

 as substrate. 



The effect of salts in accelerating the hydrolysis of casein by 

 trypsin is, in some instances, very marked. Thus in a solution 

 of "neutral" calcium caseinate containing 143 milligrams of 

 casein in 200 cc. plus 1 cc. of a 0.025 per cent solution of Gruebler's 

 trypsin at 36.5 degrees the velocity-constant of hydrolysis was 

 11 (calculated, in arbitrary units, from the monomolecular for- 

 mula) while, upon the addition of 0.05 N NaCl the value of the 

 constant rises to 44 (in the same units). 



The influence of the concentration of the added NaCl upon 

 the acceleration of the tryptic hydrolysis of caseinates which 

 it brings about is of great interest, since, so far as my observa- 

 tions have extended, it is of the same nature as its influence 

 upon the degree of swelling which gelatin plates undergo when 

 immersed in water. It will be recollected (Cf. Chap. XII) that 

 when gelatin plates are immersed in solutions of NaCl of vary- 

 ing concentration, the degree of swelling which they attain after 

 a given period does not increase continuously with the concen- 

 tration of the solution but exhibits marked maxima and minima 

 at definite concentrations (94). Exactly similar phenomena 

 occur when NaCl in varying concentration is added to a tryptic 

 digest of casein. The acceleration in hydrolysis which is pro- 

 duced by the added NaCl does not continuously increase with 

 increasing concentration of the NaCl, but exhibits marked max- 

 ima and minima. 



6. The Action of the Coagulating Ferments, Rennet and 

 Thrombin. — As is well known, rennet occurs, together with 

 pepsin, in the gastric juice and in the juices of many plants and 

 brings about the clotting of milk through the transformation 

 of casein into paracasein. Thrombin occurs in some part of the 

 circulating blood, possibly the leucocytes, and brings about the 

 coagulation of blood through the transformation of fibrinogen 

 into fibrin. The modes of action of these ferments have been 

 the subject of very many investigations (102) (103) (31) (84) 

 (75) (61), which cannot be dwelt upon in detail here. In brief, 

 however, it may be stated that rennet converts casein into 

 paracasein, whether calcium salts are present or not, and the 

 paracasein having been formed (which is soluble in the absence 

 of salts of the alkaline earths), it is coagulated (or precipitated; 



