PROTEINS AND THEIR CLASSIFICATION. 1013 



6. Adamkiewicz's reaction. A mixture is made of one volume of con- 



centrated sulphuric and two volumes of glacial acetic acid; if the 

 protein solution is added to this mixture and warmed a reddish- 

 violet color is obtained. According to Hopkins and Cole, the reac- 

 tion depends upon the presence of glyoxylic acid in the acetic acid, 

 but according to Homer it is really a formaldehyd reaction. The 

 action of the sulphuric acid upon the glyoxylic acid produces some 

 formaldehyd. This reaction seems to be due to the tryptophan 

 grouping in the protein molecule. 



7. Liebermann's reaction. Dry protein purified with alcohol and ether 



gives a blue color upon boiling with strong hydrochloric acid. 



8. The lead sulphid reaction. The protein solution is boiled with a 



solution of a lead salt made strongly alkaline with soda or potash. 

 A black precipitate or black or brown coloration results, according 

 to the amount of protein. The color is due to the splitting off of 

 sulphur and formation of lead sulphid. It is given, therefore, by 

 the sulphur-containing groups in the protein molecule. 



9. The Molisch reaction. A few drops of an alcoholic solution of a- 



naphthol are added to the protein solution and then strong sul- 

 phuric acid. A violet color is obtained. This reaction is given 

 by the carbohydrate grouping in the protein molecule. The strong 

 acid forms furfurol from this group, which then reacts with the naph- 

 thol. The reaction is not given by those proteins that do not con- 

 tain a carbohydrate group. 



Classification of the Proteins. No classification of the proteins has 

 been proposed which is entirely satisfactory. Eventually a classification 

 may be obtained based upon the chemical structure of the various proteins, 

 the number and arrangement of the constituent amino bodies, but our know- 

 ledge at present is much too incomplete for this purpose. We must be con- 

 tent with a less satisfactory system based in part upon empirical reactions 

 which have gradually been recognized in the course of physiological inves- 

 tigations. 



In the following classification the recommendations are followed of the 

 Joint Committee on Protein Nomenclature appointed by the American Physi- 

 ological Society and the American Society of Biological Chemists ("American 

 Journal of Physiology, Proc. Physiol. Soc.," vol. xxi., 1908): 



( Albumins. 



TO- i , / * u Globulins 



I. Simple proteins (protein sub- 



atives on hydrolysis). 



proteins (prolamines) - 



Histons. 



Protamins. 



II. Conjugated proteins (sub- f Glycoproteins. 

 stances which contain the pro- j Xucleoproteins. 

 tein molecule united to some -{ Hemoglobins (chromoproteins). 

 other molecule or molecules j Phosphoproteins. 

 otherwise than as a salt). [ Lecithopoteins. 



f Primary protein derivatives "] 



(formed through hydrolytic | Proteans. 

 changes which cause only J- Metaproteins. 

 slight alterations of the pro- I Coagulated proteins. 

 III. Derived proteins tein molecule). 



Secondary protein deriva- "1 T> , 

 lives. (Products of further I 

 hydrolytic cleavage of the j 

 protein molecule.) 



The Albumins. In addition to the albumins found in the cellular tis- 

 sues, the cell albumins, the conspicuous examples of this group are serum- 



