150 CYSTIN. 



tests is however not easy unless a fair amount of material is at hand, 

 and that in a pure condition. In the latter case the following tests 

 may be applied, (i) When carefully heated to 170 leucin sublimes and 

 yields a characteristic odour of amylamin. The only other substance 

 of physiological importance ordinarily met with which yields a sublimate 

 on heating is hippuric acid, due to its decomposition and the sublima- 

 tion of the benzoic acid thus set free, (ii) Sclierer's test. Only 

 applicable to very pure leucin. The suspected substance is evaporated 

 carefully to dryness with nitric acid on the lid of a platinum crucible ; 

 the residue, if it is leucin, will be almost transparent and turn yellow 

 or brown on the addition of caustic soda. If this be again very care- 

 fully concentrated with the alkali an oily drop is obtained, which runs 

 over the platinum in a spheroidal state. 



The optical properties of leucin have not as yet been fully worked 

 out. Experiment shows that its solutions are sometimes optically 

 active, at other times inactive, dependently upon the source and mode 

 of formation of the leucin. This corresponds to the expectations as to 

 its optical behaviour, based, in accordance with the Van't Hoff-Le Bel 

 hypothesis, upon its constitutional formula 1 . 



The possible relationship of leucin to the formation of urea in the 

 body has been already pointed out ( 488). It will be further con- 

 sidered under urea. 



AMIDO-AOIDS OF THE LACTIC SERIES. 



Cystin. (C 3 H 6 NSO 2 ) 2 . [S . C (CH 3 ) (NH 2 ) . COOH] 2 . Amido- 

 sulpholactic acid 2 . 



Is the chief constituent of a rarely occurring urinary calculus in 

 men and dogs. It may also occur in renal concretions, and in gravel, 

 and is occasionally found in urine, from which it separates out as a 

 greyish sediment on standing. It is prepared from this sediment, or 

 better still from cystic calculi, by solution in ammonia. This solution 

 is then allowed to evaporate spontaneously and yields the cystin in 

 regular, colourless, six sided tables of very characteristic appearance. 

 Cystin may be separated from urine by taking advantage of the 



1 For details see Mauthner, Zt. f. physiol. Chem. Bd. vn. (1882-83), S. 222. 

 Schulze, Ibid. Bd. ix. (1885), S. 100. Lewkowitsch, Ber. d. d. chem. Gesell. 1884, 

 S. 1439. Lippmann, Ibid. S. 2835. Schulze u. Bosshard, Zt. f. physiol. Chem. 

 Bd. x. (1886), S. 134. 



2 The constitution of cystin has been variously stated by different authors and 

 will only be known with certainty when its synthesis has been accomplished. 

 Slightly different formulae have been assigned to it, containing respectively 5, 6 and 

 7 atoms of hydrogen. The literature is fully quoted by Kiilz, Zt. f. Biol. Bd. xx. 

 (1884), S. 1. Cf. Baumann, Zt. f. physiol. Chem. Bd. vin. (1884), S. 299. 



