ALBUMINS OR PROTEINS. 157 



of alkaline sulphides were split off. 1 Fleitmann 2 was the first to observe 

 that only a part of the sulphur was split off by the action of alkalies, while 

 another portion remained .unacted upon. From these observations he 

 differentiated between oxidized and un-oxidized sulphur in albumin. 

 The latter, only, was susceptible of being split off. This distinction later 

 on caused much misunderstanding among investigators of the sulphur 

 components of albumin. A. Kriiger 3 is entitled to much credit for having 

 shown the futility of this classification. He makes a distinction between 

 " loosely- " and " firmly- " combined sulphur. Fr. N. Schulz 4 proved 

 that such a distinction was more justifiable, by showing that one of the 

 sulphur cleavage-products of albumin, cystine, only gave off part of its 

 sulphur on boiling with alkali; in fact, little more than half. Various 

 albuminous substances, such as keratin (from the horn of cattle, and 

 human hair), serum-albumin, and serum-globulin, acted in the same 

 manner as cystine. Morner 5 then succeeded in obtaining large quantities 

 of cystine from the above-mentioned proteins, and showed that this amino 

 acid is probably the only sulphur compound present. Other proteins cer- 

 tainly contain other sulphur compounds besides cystine. 



Among the decomposition products containing sulphur, cystine is the 

 only one that has been positively identified. Wollaston, 6 in 1810, first 

 isolated this from a renal calculus. Since then it has been separated 

 also from organs. Kiilz 7 first isolated it from digestive solutions of fibrin, 

 and Emmerling 8 found it in horn. The wide distribution of cystine as a 

 decomposition product of albumin is now generally acknowledged. 



The constitution of cystine has only recently been established; it is an 

 a-diamino-/?-dithio-dilactylic acid : 



CH 2 S S CH 



*CH.(NH 2 ) *CH(NH 2 ) 



COOH COOH 



Cystine 



1 The sulphur content of the proteins was at one time a factor of great importance 

 in the views prevailing concerning their constitution. Cf. E. Friedmann: Ergeb. 

 Physiol. (Asher and Spiro) 1, 15 (1902). E. Abderhalden: Biochem. Zentr. 2, 257 

 (1904). 



T. Fleitmann: Ann. 61, 121 (1847); 66, 380 (1848). 



A. Kriiger: Pfliiger's Arch. 43, 244 (1888). 



Fr. N. Schulz: Z. physiol. Chem. 25, 16 (1898). 



K. A. H. Morner: ibid. 28, 595 (1899); 34, 207 (1901-02). 



Wollaston: Phil. Trans. 1810, 220. 



E. Kiilz: Z. Biol. 27, 415 (1890). 



O. Emmerling: Verh. Ges. Naturforsch Aerzte. 2, 391 (1894). 



