PROTEINS 101 



PRECIPITATION REACTIONS AND OTHER PROTEIN TESTS 



There are three forms in which proteins may be precipitated, i.e., 

 unaltered, as an albuminate, and as ah insoluble salt. An instance of the 

 precipitation in a native or unaltered condition is seen in the so-called 

 salting-out experiments. Various salts, notably (NH^SO^ ZnS04, 

 MgS04, Na2S04 and NaCl, possess the power, when added in solid form 

 to certain definite protein solutions, of rendering the menstruum incap- 

 able of holding the protein in solution, thereby causing the protein to be 

 precipitated or salted-out, to use the common term. Mineral acids and 

 alcohol also precipitate proteins unaltered. In the case of concentrated 

 acids the protein is dissolved in the presence of an excess of acid with 

 the formation of a protein salt. Proteins are precipitated as albu- 

 minates when treated with certain metallic salts, and precipitated as 

 insoluble salts when weak organic acids such as certain of the alkaloidal 

 reagents are added to their solutions. 



If certain acids (picric, phosphotungstic, phosphomolybdic, tannic, 

 or chromic) be added to a neutral albumin solution, a precipitate of an 

 insoluble protein salt occurs. If, however, the salts of these acids be 

 added no precipitate occurs. The addition of a small amount of acid, 

 as acetic acid, to such a solution will cause a precipitate to form. 1 



The effect of the addition of the salts of the heavy metals is in the 

 first instance to cause a precipitation of the protein. In many cases, 

 however, the addition of an excess of such salts causes the solution of 

 the precipitate, while a further excess may cause a reprecipitation. The 

 precipitate which is first formed in a protein solution by the addition 

 of the salts of the heavy metals may be redissolved not only by an 

 excess of such salts but by an excess of protein as well. 2 



Colloidal iron, kaolin and alumina cream are frequently used for 

 removing proteins from solution. The process is one of adsorption 

 and has been adapted to certain quantitative methods. 



It is generally stated that globulins are precipitated from their solu- 

 tions upon half saturation with ammonium sulphate and that albumins 

 are precipitated upon complete saturation by this salt. Comparatively 

 few exceptions were found to this rule until proteins of vegetable origin 

 came to be more extensively studied. These studies, furthered es- 

 pecially by Osborne and associates, have demonstrated very clearly 

 that the characterization of a globulin as a protein which is precipitated 

 by half saturation with ammonium sulphate, can no longer hold. 

 Certain vegetable globulins have been isolated which are not precipi- 



x Mathews: Amer. Jour, of Physiology, i, 445* 1898. . . 



2 Pauli: Hofmeister's Beitrage, 6, 233, 1904-05; Robertson: Ergebmsse der Physiolo&e, 

 10, 290, 1910. 



