2 34 Journal of Agriculture. [8 April, 1907- 



SERUM ALBUMEN in blood, whilst EGG ALBUMEN Or OVALBUMEN COnstitUtCS- 



the chief solid constituent of white of egg. Albumens are soluble in 

 water, and are coag-ulated by heat as is familiar to us. in the cooking of 

 an egg. 



GLOBULINS. — Globulins accompan\ albumens wherever these are 

 found. The chief protein in muscle is a globulin, and the more important 

 vegetable proteins belong also to this class. Globulins, like albumens, are 

 coagulated by heat, witness the cooking of meat, but, unlike albumens, 

 are insoluble in pure water. The addition of a small amount of salt to 

 the water renders them soluble, and in tliis wav thev can appear in 

 bioplasm, blood. &c., in dissolved form. 



PHOSPHO-PROTEINS.— These are more complex than the two pre- 

 ceding, and, as the name implies, are rich in phosphorus. The most 

 important phospho-protein is caseinogen,^ the chief protein in milk. 

 Caseinogen is insoluble in water and dilute acids, but soluble in weak 

 alkalies. If to a solution in weak alkali sufficient acid be added, the 

 caseinogen is thrown out as a precipitate. This explains the curdling of 

 milk when it sours or when acid is deliberately added to it.^ Caseinogen 

 in solution is not coagulated by boiling. 



Another imjjortant phospho-protein is vitellin, which is found in yollc 

 of egg. 



NUCLEO-PROTEINS. — These are present in large amount in the 

 nuclei, and, to a lesser extent, in protoplasm ; they are also found in 

 certain body-fluids, particularly in the bile of the ox. 



The structure of the nucleo-proteins is extremely complex, as no less 

 than two complete protein molecules are linked on to a complex acid called 

 nucleic acid. This nucleic acid is itself fairly complex in structure, and^ 

 though not comparable with protein, has nevertheless several groups of 

 bodies present, amongst which we may note a sugar, phosphoric acid, and 

 some compounds of carlx>n. hydrogen, oxygen, and nitrogen, which are 

 closely allied to uric acid. Nucleic acid united with one protein grou]> 

 gives NUCLEiN, with two protein groups, nucleo-protein. 



The nucleo-proteins have many properties in common with caseinogen. 

 for example, thev are not coagulated by heat, are insoluble in water and 

 weak acids, but soluble in weak alkalies. 



GLUCOPROTEINS. — These are compounded of protein and a 

 carbohvdra.te. The chief glucoprotein is mucin, the glairy adhesive 

 material which is found in the inner wall of most canals and ducts of the 

 bodv. It is present also in large quantity in the slime of snails and in the 

 saliva of most mammals. If to saliva we add some dilute acid, the mucin 

 is thrown out as a stringy precipitate, which can be redissolved by adding 

 sufficient alkali. The solubilities of mucin are the same as those of 

 caseinogen and nucleo-proteins. 



CHROMOPROTEINS'.— The chief chromoprotein is hemoglobin, 

 which is a compound of protein and a substance called haematin. This- 



' An unfoi'tuiiate confusion exists as to the nomenclature of tills protein. In Englaml flic term 

 caseinogen is used for the chief protein of milk, and casein is restricted to the product of rcnnin il' it ting ; 

 in Germany, kasein is used for the former and parakasein for the latter. The Americans and some 

 English chen;ists follow the German nomenclature. In accordance with the views of tiie committee 

 appointed jointly by the Chemical and Physiological Societies of England the term CASEIN is used for 

 the rennin product and caseinogen for the unaltered protein as it occurs in milk. 



^ The reader must bear in mind the fundamental difference between the clotting of milk by an 

 acid and the clotting prorUiced by ferments, such as reraiin. In the former case, the caseinogen is 

 rendered insoluble by the acid, but is not chemically altered and can be brought back again into the 

 soUible form. With remiin a distinct chemical transformation occurs, and the product casein camiot 

 by any means be made to revert to caseinogen. 



