ISOLATION AND PREPARATION 19 



the preparations which he obtained from seeds by neutralising alkaline 

 extracts were soluble in sodium chloride solutions, and others have also 

 found that, in many cases, the protein thus dissolved could be obtained 

 in the same crystalline form and with the same chemical and physical 

 properties as after direct extraction with sodium chloride solutions. 

 It is, therefore, almost certain that, if the concentration in alkali does 

 not exceed a proper limit, many, if not all, seed proteins can be obtained 

 by its use with unchanged properties. 



The greatest danger in employing this method comes not from the 

 action of the alkali but from that of the acid which is used in neutralis- 

 ing it. In precipitating the proteins from alkaline extracts a complete 

 separation is usually obtained only when an excess of acid is added 

 beyond that required to neutralise the alkaline, or in other words, only 

 when sufficient acid is added to form a salt of the protein insoluble in 

 water. In neutralising alkaline extracts it is exceedingly difficult to 

 add just enough acid to effect precipitation without exceeding this 

 amount. If the excess of acid be even exceedingly small the protein 

 is rapidly converted into products which are no longer soluble in neutral 

 saline solutions. (Compare Chapter VII., p. 39.) 



The amount of protein extracted by alkalies is usually greater than 

 that extracted by neutral saline solutions or by water, in fact, in many 

 cases it is very much greater. Sufficient attention has not yet been 

 directed to the cause of this difference, and for this reason our present 

 knowledge of the character of the total protein constituents of the 

 greater number of seeds which have been studied is still incomplete. 

 This difference may be due to any one of several causes. The alkali 

 may dissolve protein which in its native condition is insoluble in salt 

 solutions but soluble in alkali. It may dissolve protein enclosed within 

 unruptured cells and hence inaccessible to the action of neutral solvents. 

 It may dissolve compounds of the globulin with non-protein substances 

 which are insoluble in neutral solvents. The precipitate produced by 

 neutralisation may consist of, globulin which undergoes changes during 

 the process of extraction with and isolation from salt solutions whereby 

 a large part of the original globulin is converted into proteoses which 

 on dialysis are lost by diffusion, or even into polypeptide combinations 

 or free amino-acids which are not precipitable by saturating their solu- 

 tions with ammonium sulphate, and are thus lost in the ordinary pro- 

 cesses employed for isolating the dissolved protein. 



According to the older view the proteins soluble in alkalies dis- 

 solved in consequence of the formation of soluble alkaline salts, hence 

 these proteins were designated caseins. Although protein may some- 



