150 BIOLOGICAL CHEMISTRY 



H 8 C S S CH CH 8 SH ^S 



ill CH. 



CHNH 2 CHNH t CHNH, 



I I I CH 2 NH, 



COOH COOH COOH 



Cystine. Cysteine. Taurine. 



Some of the sulphates are combined with organic groups to 

 form ethereal sulphates, so we have to distinguish between 

 the inorganic and ethereal sulphates. The ethereal sulphates 

 are formed as a means of protection against certain poisonous 



S0 8 S0 2 



>OH 



Sulphuric acid. Ethereal sulphuric acid. 



substances. The formation of phenol, skatol, indol, etc., in 

 the intestine may lead to absorption of these substances, which 

 will then exert poisonous action on the cells of the body. 

 These substances are rendered non-toxic by combining them 

 with sulphuric acid to form ethereal sulphates. The ethereal 

 sulphates are not precipitated by barium salts, or by benzidire, 

 hence the inorganic sulphates can be estimated without the 

 ethereal sulphates. By boiling with acid the ethereal sulphates 

 are hydrolysed, setting free the organic substance and sulphuric 

 acid. The latter will then give a precipitate with barium 

 salts. The ethereal sulphates are usually estimated by 

 taking the difference between the inorganic sulphates and the 

 sulphates after hydrolysing with acid.* 



Neutral sulphur is also found in the excreta. It consists 

 of sulphur not in the form of sulphates. In certain individuals 

 the sulphur metabolism is defective and hexagonal crystals of 

 cystin are deposited in the urine. f 



For further observations on sulphur, see sulphur bacteria, 

 p. 211. 



PHOSPHORUS METABOLISM 



Phosphorus is excreted in the form of phosphoric acid. 

 The phosphorus is derived from phosphates in the food, 

 nucleo-proteins, phospho-proteins and phospho-lipins. 



The discussion of metabolism refers specifically to mammals, 

 but there is no doubt that similar relations hold for all living 



* For Gravimetric Estimation see O. Folin, Journ. Biol, Chem., 

 1906, vol. i, p. 131 ; for Volumetric Estimation, see O. Rosenheim and 

 J. C. Drummond, Biochem. Journ., 1914, vol. 8, p. 143. 



t A. E. Garrod, Inborn Errors of Metabolism. Oxford, Clarendon 

 Press, 1909. 



