22 PKOTEIDS. 



The question as to the identity or the reverse of casein and alkali- 

 albumin as obtained by the action of alkalis on other proteids has 

 given rise to much controversy. Some authors have considered them 

 to be identical 1 , but that they are not so is sufficiently shewn by the 

 following facts. Solutions of alkali-albumin cannot be made to clot by 

 the action of pure rennin. If milk sugar be added to the solution and 

 impure rennet, i.e. extract of the mucous membrane containing rennin, 

 be allowed to act upon it, in some cases a separation of the alkali- 

 albumin may take place owing to the formation of lactic acid which 

 then precipitates the albumin. In the absence of the milk sugar no 

 change is produced which can in any way be regarded as analogous to 

 the clotting of casein. When milk is clotted the separation of the 

 casein is so complete that none is found in the ' whey,' and Hammar- 

 sten has shewn that if alkali-albumin be added to milk and the mixture 

 be then clotted, alkali-albumin may be obtained from the whey on break- 

 ing up the curd. It has further been shewn 2 that although casein is 

 very resistant to the action of acids, it may by treatment with them 

 be converted into acid-albumin with complete loss of all clotting powers, 

 and still more readily into alkali-albumin by the action of alkalis. 



A further difference of the two substances was urged by Zahn on the basis of his 

 experiments on the nitration of milk through porous earthenware (battery-cells) 3 . 

 He found that solutions of alkali-albumin pass through the walls of the cells as 

 rapidly as do solutions of serum-albumin: when milk however is filtered by this 

 method, casein does not pass and the filtrate consists of water, salts and the 

 coagulable proteid of the milk. Whether this indicates any difference between the 

 two substances is however doubtful, for it is still an open question whether casein 

 is truly in solution in milk. Further it is stated that the casein also passes into 

 the filtrate if the filtration is prolonged 4 , and Soxhlet states that if finely divided 

 (emulsified) fat be suspended in a solution of alkali-albumin the filtration of this 

 substance is rendered as impossible as that of casein in milk. 



The crucial distinction between the two substances is the fact that 

 casein can be clotted by rennin with simultaneous formation of a soluble 

 proteid by-product, whereas no true clot can ever be obtained from 

 ordinary alkali-albumin. 



After the removal of casein from milk by precipitation, the nitrate 

 contains a small amount of coagulable proteid sometimes spoken of as 

 'lactalbumin,' closely resembling serum-albumin in its general properties, 

 but differing slightly as to its specific rotatory power and the tempera- 

 ture at which it coagulates when heated 5 . 



1 Soxhlet, loc. cit. 2 Lundberg, loc. cit. 



3 Zahn, Pfliiger's Arch. Bd. n. (1869), S. 598. 



4 Schwalbe, Centralb. f. d. med. Wiss. 1872, S. 66. 



5 Sebelien, Zt. /. physiol Cliem. Bd. ix. (1885), S. 445, xm. (1889), S. 135. 

 Eugling, see Maly's Bericht. Bd. xv. (1885% S. 183. Halliburton, Jl of Physiol. 

 Vol. xi. (1890), p. 451. 



