146 THE PROTEIN SUBSTANCES. 



d-glutamic acid forms a beautifully crystalline combination with hydro- 

 chloric acid, which is nearly insoluble in concentrated hydrochloric acid. 

 This compound is used in the isolation of glutamic acid. On boiling 

 with cupric hydroxide a beautiful crystalline copper salt, which is soluble 

 with difficulty, is obtained. The benzoyl-d-glutamic acid melts at 

 130-132 C. The hydrochloride, the a-naphthylisocyanate of glutamic 

 acid, which melts at 236-237 C., the analysis of the free acid, and the 

 specific rotation are used in its detection. 



As previously stated monamino-oxydicarboxylic acids have also 

 been found among the cleavage products of the proteins. To these belong 

 the following: 



That oxyaminosuccinic acid, C U H 7 NO 5 occurs among the hydrolytic cleavage 

 products of proteids has been shown to be probable by SKRAUP. This acid has 

 been prepared synthetically by NEUBERG and SILBERMANN from diaminosuccinic 

 acid and barium nitrite -in sulphuric acid solution. Oxyaminosuberic acid, 

 C 8 H 15 NO 5 , has been detected by WOHLGEMUTH l in the cleavage products of a 

 liver nucleoprotein. 



Cystine, C 6 H 12 N 2 S2O4 the disulphide of cysteine (a-amino-/?-thiolactic 



CH 2 O O CH 2 



acid), CH(NH 2 ) CH(NH 2 ), was first obtained with certainty as a 



COOH COOH 



cleavage product of protein substances by K. MORNER, and then also 

 by EMBDEN. KULZ 2 obtained it once as a product of tryptic digestion 

 of fibrin. The quantities found by MORNER and BUCHTALA in the 

 various proteins are given in the tables on pages 106, 107, 113 and 114. 



According to NEUBERG and MAYER 3 two kinds of cystine occur in nature, 

 namely, stone-cystine, designated ,fl-cystine, and protein-cystine. Stone-cystine 



CH 2 NH 2 CH 2 NH 2 



is the disulphide of ( 5-amino-a-thiolactic acid, CH S S CH 



COOH COOH 



The protein-cystine has been chiefly obtained from the protein substance, 

 but also from calculi, while the stone-cystine has been obtained from urinary 

 calculi only. 



Many objections have been raised from many sides as to the correctness of 

 this assumption. ROTHERA could not find any difference between the stone- 

 cystine and the cystine prepared from hair, and FISCHER and SUZUKI, and recently 

 also ABDERHALDEN, 4 arrived at similar results, which seems to place the exist- 

 ence of stone-cystine in doubt. The occurrence of two structurally isomeric cys- 

 tines is not improbable, from certain observations of MORNER, but FRIEDMANN 

 and BAER 5 have shown that these observations do not lead to this assumption^ 



1 Skraup, Zeitschr. f. physiol. Chem., 42; Neuberg and Silbermann, ibid., 44; 

 Wohlgemuth, ibid., 44. 



2 K. Morner, ibid., 28, 34, and 42; Embden, ibid., 32; Kiilz, Zeitschr. f. Biologic, 27. 



3 Zeitschr. f. physiol. Chem., 44. 



4 Rothera, Journ. of Physiol., 32; Fischer and Suzuki, Zeitschr. f. physiol. Chem., 

 45; Abderhalden, ibid., 51. 



5 Friedmann, Hofmeister's Beitrage, 3. With Baer, ibid., 8. 



