ELEMENTARY PHYSIOLOGICAL CHEMISTRY 175 



CHAPTER IV. 

 PROTEIDS CONTINUED. 



CLASSIFICATION or PROTEIDS. 



ON account of the fragmentary nature of our knowledge regarding 

 the chemical constitution of proteids, it is at present impossible to 

 classify them with anything like the same precision as is the case with 

 carbohydrates. As noted above, Kossel has suggested that all proteids 

 contain as their basis of construction (or nucleus) the substance protamin 

 and that to this are attached different side groups, such as mon- 

 amido acids, aromatic bodies, sulphur, etc., the nature and the relative 

 amounts of these side groups determining the kind of proteid. 



It has been found, however, on putting this proposed classification to 

 the test, that certain proteids (e.g. elastin) do not contain the three 

 hexone bases necessary for the production of protamin, so that it will 

 be necessary to adopt at present the older classification of Drechsel, 

 adding to it a new group containing the protamines. 



The following classification is modified after Drechsel and Kossel : 



I. Protamines (C 80 H 5r N 17 6 ) contain only the hexone bases, and 

 exist, combined with nucleic acid, in the spermatozoa of certain fishes. 



II. Albuminoids contain hexone bases plus mon-amido acids, such 

 as leucin (C 6 H 13 N0 2 ), along with a variable amount of sulphur. 

 Certain of the members of this group also contain traces of aromatic 

 radicles, such as tyrosin (C 9 H U N0 3 ). The chief representative of this 

 group is collagen, which, on boiling with water, is changed into 

 gelatine. 



EXPERIMENT XII. Take a piece of gelatine ; dissolve it in a test- 

 tube full of warm water; divide the resulting solution into three 

 parts, a, b, and c; allow a to cool, when the gelatine solution will 

 set into a jelly. 



To b apply the biuret reaction; a violet colour is produced. 



To c apply Millon's test ; a very faint red is produced on boiling, as 

 gelatine only contains a trace of an aromatic radicle on which this 

 reaction depends. Some observers consider that the aromatic radicle 

 is not really built up in the gelatine molecule, but is only attached to 

 it as an impurity. 



The other important albuminoids are keratin (the chief constituent 

 of epidermis, hairs, nails, etc.) and elastin (composing elastic ligaments). 

 (See Advanced Course.) 





