Il6 PHYSIOLOGICAL CHEMISTRY 



fibrinogen (see page 258), the coagulation may be produced by enzyme 

 action. Ordinary soluble proteins after having been transformed into 

 the coagulated modification are no longer soluble in the ordinary sol- 

 vents. Upon being heated in the presence of strong acids or alkalis, 

 coagulated proteins are converted into metaproteins. 



Many proteins coagulate at an approximately fixed temperature 

 under definite conditions (see pages 104 and 357). This characteristic 

 may be applied to separate different coagulable proteins from the same 

 solution by fractional coagulation. The coagulation temperature fre- 

 quently may serve in a measure to identify proteins in a manner similar 

 to the melting-point or boiling-point of many other organic substances. 

 The separation of proteins by fractional coagulation is thus analogous 

 to the separation of volatile substances by means of fractional distillation. 

 This method of separating proteins is not a satisfactory one, however, 

 inasmuch as proteins in solution have different effects upon one another 

 and also because of the fact that the nature of the solvent causes a 

 variation in the temperature at which a given protein coagulates. The 

 nature of the process involved in the coagulation of proteins by heat 

 is not well understood, but it is probable that in addition to the altered 

 arrangement of the component atoms in the molecule, there is a mild 

 hydrolysis which is accompanied by the liberation of minute amounts 

 of hydrogen, nitrogen, and sulphur. The presence of a neutral salt 

 or a trace of mineral acid may facilitate the coagulation of a protein 

 solution (see page 104), whereas any appreciable amount of acid or 

 alkali will retard or entirely prevent such coagulation. 



It has been shown that the coagulation of proteins by heat pro- 

 ceeds in two stages: 1 first, a reaction between the protein and the hot 

 water (denaturation) , and second, an agglutination or separation of the 

 altered protein in particulate form. The concentration of acid, or 

 hydrogen ion, in the solution influences the coagulation of proteins, such 

 that the original protein is acted upon less readily by hot water alone 

 than in the presence of acid. The formation of the coagulum is ac- 

 companied by the disappearance of the free acid from the solution, 

 indicating the formation of a protein salt. A disturbance of the equi- 

 librium between the hydrolyzed and unhydrolyzed portions of the pro- 

 tein salt, due to the greater rapidity with which the unhydrolyzed 

 portion is precipitated, results in the gradual removal of both pro- 

 tein and acid from the solution. This has been offered as an explana-* 

 tion of the decreasing acidity. 



According to Chick and Martin, the addition of neutral salts to the 

 acid solution of the salt-free protein to be coagulated results in a decreased 



1 Chick and Martin- Journal of Physiology, 43 i, 1911. 



