148 



ESSENTIALS OF CHEMICAL PHYSIOLOGY 



5. Among the important reactions of proteids is Piotrowski's reaction — 

 that is, the coloration produced by copper sulphate and a caustic alkali ; the- 

 term ' biuret reaction ' is applied to the rose-red colour which proteoses and 

 peptones give with these reagents, because biuret (a derivative of urea) give* 

 a similar colour. It does not, however, prove that biuret is contained in the 

 proteid molecule. Biuret and proteid both contain some radicle to which 

 the colour is due. Gnezda ' thought it was cyanogen, and that the cyanogen 

 was differently combined in the peptones and native proteids (albumins and 

 globulins) respectively ; hence the rose-red given by one group and the violet 

 by the other. More recent work by Pickering,^ however, points to a CONH 

 group rather than cyanogen. Gnezda found that if a dilute solution of 

 nickel sulphate is used instead of copper sulphate, the native proteids give 

 different colours from the peptones and proteoses, and Pickering has foimd 

 the same with cobalt. Their results may be given in the following 

 table : — 



6. Another deHcate test introduced by Mc William may here be mentioned r 

 Salicyl-sulphonic acid precipitates albumins and globulins : on heating the 

 precipitate is coagulated. The same reagent precipitates proteoses. On 

 heating the precipitate dissolves and reappears on cooling. It does not 

 precipitate peptones. 



7. The use of trichloracetic acid for the separation of various proteid* 

 may be illustrated by the following experiment. Take some blood and add 

 to it some solution of Witte's peptone (i.e. proteoses and peptone). Add to 

 this mixture an equal volume of a 10-per-cent. solution of trichloracetic acid. 

 There is an abundant precipitate. Boil rapidly and filter hot. The filtrate 

 contains the proteoses and peptone, all the other proteids being contained in 

 the precipitate. On cooling, the filtrate deposits some of the proteose. The 

 proteose and peptone may be detected in the usual way. 



' Proc. Roy. Society, vol. xlvii. p. 202. 



' Journal of Physiology, vol. xiv. Most of the other colour reactions of pro- 

 teids depend on the aromatic radicle they contain. 



