THE CHEMISTRY OF THE AS IMA L BODY. 547 



NH 2 

 CH 3 — C — COOH. It is a product of proteid metabolism and is normally 



SH 



destroyed in the body. On the introduction of a halogen derivative of benzol 

 into the body, compounds are formed with cyste'in, called mercapturic acids, 

 which appear in the urine : 



NH 2 NH 2 



CH 3 -C— COOH + C,H 5 Br + O = CH 3 — C— COOH + H 2 0. 



I I 



8H SC 6 H 4 Br. 



Broraophenyl-mereapturic acid. 



This proves that cystei'n (like glycocoll, for example) is at least an intermediary 

 and possibly a primary product of proteid metabolism, [f cyste'in be fed, the 

 greater part (two-thirds) of the sulphur appears in the urine as sulphuric acid, 

 the rest as neutral sulphur. Thiolactic acid has been found 1 as a decomposition 

 product of horn. Baumann 2 demonstrates the reduction of cyste'in to thiolactic 

 acid, shows that the latter yields an odor of ethyl sulphide on evaporation, 

 and asks if thiolactic acid be not the mother substance of Abel's compound 

 (see p. 507) : 



NH 2 

 CH 3 — C— COOH + H 2 = CH 3 CH(SH)COOH + NH 3 . 



Thiolactic acid. 



SH 



Cyste'in itself is never directly detected in the urine or in the body. 



Cystin, Dithio-diamido-ethidene Lactic Acid. — Cyste'in is converted by 

 atmospheric oxygen into cystin : 



NH, 



2CH — C— COOH + 20 = 



CH 3 — CSNH 2 — COOH 



CH 3 — CSNH— COOH 



S 'I Cystin. 



Cystin is very insoluble in water. In particular cases it appears in considerable 

 quantities as a urinary sediment, still more rarely as a stone in the bladder 

 (see p. 543). It has been detected in the normal livers of horses.' It is 

 Iaevo- rotatory. 



It is reported * that bodies having the composition (' S II ithio- acids, meivaptans) 

 may form sulphuric acid, while most of those having the composition — S — C 

 (ethyl sulphide) are not oxidized in the body. 



1 Suter: ZeUaehrift fur physiologische Chemie, 1895, Bd. 20, S, 564. 



2 Baumann: Tbid., 1895, Bd, 20, 6. 583. 



3 Drechsel : Zeitsehrift fur Biologie, 1897, Bd. 33, S. B5. 



* W. .J. Smith : Pfliiger's Archiv, 1894, Bd. -V), S. 542, and 1894, Bd. 57, S. H8. 



