S April, 1907] Eleiiicuts of Aii/i/tal Physiology. 231 



THE ELEMEXT8 OF AMMAL PHYSIOLOGY. 



IF. A. Osborne, M.B., D.Sc, Professor of PhysioIoi;v and Histology, 

 Dean of tJie Faculty of Agriculture in ilie Uni-'crsitv of Melbourne. 



{Continued from page 8g.) 



CHAPTER V. 



The Proteins. 



We have seen in the preceding chapter that complex polvsaccliarides, 

 •such as starch, are formed by the union of many units of a comparatively 

 simple sugar, namely dextrose. But the proteins with which this paper 

 deals are immensely more complex than any carbohydrate, for they are 

 formed, not by the union of a number of units of a single substance, but 

 by the union of many units of many substances. We have seen that a 

 complex carbohydrate, when boiled with an acid, splits up into its com- 

 ponent parts ; the same occurs with a protein, and the chemist is able in 

 this manner to determine what the constituent groups are. Now% the simple 

 bodies into which proteins can be decomposed, and from which they can 

 be built up by the plant or animal, all belong tO' the class of bodies called 

 AMiNO-ACiDS. These are soluble, crystalline substances, weakly acid in 

 •character, and containing nitrogen as well as carbon, hydrogen, and oxygen. 

 Tt would be inadvisable here to give anything like a complete list of the 

 .amino'-acids which can enter into the composition of proteins ; the im- 

 portant ones only will be mentioned, and these are — ■ 



Leucin, which crystallizes in spherical lumps, and is gritty, like sand. 

 It can often be seen as a sediment in pancreatic extracts, 



Glycocoll, which has a sweetish taste and is formed in quantity when 

 gelatine is boiled with an acid, 



Tyrosin, found in considerable amount in cheese, and hence its name 

 {Greek, turos = cheese). This important amino-acid belongs to the same 

 great c^ass of chemical bodies as carbolic acid and benzoic acid;, that is. the 

 class of aromatic compounds. 



Tryptophane, this body is the mother substance of nearlv all the pig- 

 ments of the animal body : in itself it possesses no colour, but when its 

 structure is slightly modified it can yield substances of very rich tints — 

 the colour -of blood, bile, of urine, all being due to this body; even' tihe 

 green colouring matter in plants is a close ally. Moreover, it is largely 

 to tryptophane that the evil smell of a putrid animal bodv is due, as also 

 the smell of the faeces of carnivores, for tryptophane, under the action of 

 bacteria, can pass readily into chemical bodies (indol, skatol. &:c.), which 

 have a penetrating and disgusting odour, 



Cystin, this amino-acid contains not only carbon. h\drogen, oxvgen, 

 and nitrogen, but also sulphur, and it is to cvstin that the sulphur content 

 of protein is due. Cystin, when attacked by bacteria, gives off its sulphur 

 as sulphuretted hydrogen, a poisonous gas which some peonle regard as 

 having a most objectionable odour, and being reminiscent of rotten eggs, 



Hexone AMINO-ACIDS, thesc contain more nitrogen and have much 

 weaker acidic properties than the amino- acids mentioned. They are pre- 

 sent in all true proteins, and are obtained in considerable quantity from 

 the soft roe of fishes. 



