58 



CHEMISTRY OF THE PROTEIDS 



CHAP. 



cystin. By dissociating dihydrouracil sulphocyanate by means of 

 fuming HC1 at 170, he obtained a-thio-/2-amino glyceric acid : 



CH 2 NH 2 



ON SCH 



CO 



I M : 



CO NH ; 



Dihydrouracil sulphocyanate. 



4H 2 = 2C0 2 



CH 2 . NH 2 



2NH + CH . SH 



COOH 



Isocystin. 



Gabriel derives both a-alanin and a-thio-lactic acid (see above) from 

 protein-cystin or a-amino-/3-thioglyceric acid " disulphide " according 

 to the following scheme : 



CH 2 



\< 



-NH 



CH X 

 COOH 



CH 3 



CH.SH 



COOH 



CH 2 SH 



CH.NH 

 COOH 



-HS 



CH 2 



I 

 CH' 



COOH 



NH 



CH 3 



CH . NH 2 

 COOH. 



Historical Account. Cystin was first discovered in 1810 by 

 Wollaston 1 in a urinary calculus. Kiilz 2 found it occasionally 

 and in small quantities on digesting fibrin with trypsin ; Emmer- 

 ling 3 on dissociating horny substances with acids, but subsequently 

 it was shown by Morner 4 and Embden 5 to be a constant and 

 abundant dissociation-product of most albumins. 



Morner and Embden have also found cy stein, along with, and in 

 place of, cystin. Embden took cystein to be the primary dissociation- 

 product ; Patten, 6 however, has shown that in making cystin, a part of 

 it is converted into cystein, while the converse does not hold good-. 

 Cystin is therefore the primary dissociation-product. 



Baumann 7 first demonstrated that cystin in the urine is the disul- 

 phide of cystein. He believed the latter to be a-a-amino-thiolactic 

 acid, having the ammonia- and hydrogen-sulphide remainders attached 

 to the same carbon atom. 



1 W. H. Wollaston, Phil. Trans. 1810, pp. 223-330. 



2 E. Kiilz, Zeitschrift fur Biologic, 27. 415 (1890). 



3 A. Ernmerling, Verhandl. d. Ges. deutsch. Naturforscher u. Arzte, 1894, II. 2. 391. 



4 K. A. H. Morner, Zeitschr.f. physiol. Chem. 28. 595 (1899), 4. 207 (1901). 



5 G. Embden, ibid. 32. 94 (1900). 6 A. J. Patten, ibid. 39. 350 (1903). 

 7 E. Baumann, ibid. 8. 299 (1884). 



