68 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



8. Separation of the Individual Amino Acids. The greatest loss 

 occurs here as none of the methods of isolating the compounds are 

 perfect. 



(a) Proline. The quantity extracted by alcohol is much greater 

 than that obtained in a pure crystalline state and reckoned as pro- 

 line. An estimation by Van Slyke's method may give the actual 

 amount, though Abderhalden and Kautzsch think that this value is 

 not really satisfactory. Abderhalden and Kautzsch [1912] regard the 

 proline content determined gravimetrically as being very much below 

 the actual amount. 



() Valine, Leucine, Isoleucine. The figures given represent the 

 quantity of substance isolated in a pure state. The leucine figures 

 in the tables are really those for leucine + isoleucine. 



(c) Glycine and Alanine. Nearly all the glycine can be obtained, 

 but the actual amount of alanine is much greater. 



(d) Glutamic Acid. Probably the figures given for this amino acid 

 most nearly approach the real content of the protein in this constituent. 



(e) Aspartic Acid and Serine. The figures are much too low, as 

 the method of separation is extremely unsatisfactory. 



9. Oxyproline. The amount of this substance in the protein is 

 greater than the quantities which have been isolated. Its method of 

 isolation is so laborious that data are only available for a few proteins. 



10. Tyrosine and Cystine. The data for tyrosine most probably 

 represent the content of the protein in this unit very closely. 



The data for cystine are unsatisfactory ; in many cases the data 

 are calculated from the sulphur content of the protein ; in the other 

 cases they are those from pure isolated cystine. More satisfactory 

 data are given by Van Slyke's amino method (p. 85). 



11. Tryptophan. A large amount of protein is required for the 

 isolation and estimation of tryptophan ; it is on this account most 

 probably that so few data exist. The isolation is easier the larger the 

 quantity of material. Tryptophan might be isolated first in most cases 

 and the remainder of the products isolated subsequently. 



Osborne, allowing for all losses, calculates that from 4 1-82 per cent, 

 can actually be recovered. In the cases of zein and gliadin 86 and 68 

 per cent, respectively are known ; in the case of vignin only 5 per cent, 

 more has to "be accounted for. The losses from the esters are com- 

 puted to be 50 per cent, of the alanine, serine, aspartic acid ; 30 per 

 cent, ot the valine, proline, glutamic acid, phenylalanine ; 20 per cent, of 

 the leucine : only 50 per cent, of the cystine and tryptophan may be 

 accounted for. 



