CASEIN. 615 



Casein. This protein substance, which thus far has been detected 

 positively only in milk, belongs to the nucleoalbumins, and differs from 

 the albuminates chiefly by its content of phosphorus and by its behavior 

 with the rennet enzyme. Casein from cow's milk has the following com- 

 position: C 53.0, H 7.0, N 15.7, S 0.8, P 0.85, and O 22.65 per cent. Its 

 specific rotation is, according to HOPPE-SEYLER, rather variable; in 

 neutral solution it is (a) D = 80; its faintly alkaline solution has a 

 stronger rotation, namely, 97.8 to 111.8, in a solution of N/10 N/5 

 NaOH (LONG x ). The question whether the casein from different kinds 

 of milk is identical or whether there are several caseins cannot be decided 

 by the elementary analysis. According to TANGL and CSOKAS 2 mare's 

 and ass's casein seem to be somewhat richer in nitrogen (16.44 and 16.28 

 per cent respectively) but poorer in sulphur (0.528 and 0.588 per cent) 

 and carbon (52.36 and 52.27 per cent) than the casein from cud chewers. 

 The ass's casein was richer in phosphorus (1.057 per cent) than the mare's 

 or cow's casein (both with 0.887 per cent). 



Casein when dry appears like a fine white powder, which has no 

 measurable solubility in pure water (LAQUEUR and SACKUR). Casein 

 is only very slightly soluble in the ordinary neutral-salt solutions. Accord- 

 ing to ARTHUS it dissolves rather easily in a 1 per cent solution of sodium 

 fluoride, ammonium or potassium oxalate. ROBERTSON thinks that it 

 is more soluble in potassium cyanide and the alkali salts of certain vola- 

 tile fatty acids such as butyric acid and valeric acid than in solutions 

 of the ordinary neutral salts. It is at least a tetrabasic acid, whose 

 equivalent weight is 1135 according to LAQUEUR and SACKUR, and 1250 

 according to ROBERTSON. The statements as to the molecular weight 

 are disputed (LAQUEUR and SACKUR, L. and D. VAN SLYKE S ). It 

 dissolves readily in water with the aid of alkali or alkaline earths, also 

 calcium carbonate, from which it expels carbon dioxide. If casein is 

 dissolved in lime-water and this solution carefully treated with very 

 dilute phosphoric acid until it is neutral in reaction, the casein appears 

 to remain in solution, but is probably only swollen as in milk, and the 

 liquid contains at the same time a large quantity of calcium phosphate 

 without any precipitate or any suspended particles being visible. The 

 casein solutions containing lime are opalescent and have on warming 

 the appearance of milk deficient in fat (which is also true for the salts 

 of casein with the alkaline earths). Therefore it is not impossible that 



1 Hoppe-Seyler, Handb. d. physiol. u. pathol. chem. Analyse, 7. Aufl., 368; Long, 

 Journ. Amer. Chem. Soc., 27. 



2 Pfliiger's Arch., 121. 



3 Laqueur and Sackur, Hofmeister's Beitrage, 3; M. Arthus, Theses presentees 

 a la faculte des sciences de Paris, 1893; Robertson, Journ. of biol. Chem., 2; L. and 

 D. van Slyke, Amer. Chem. Journ., 38. 



