CHEMICAL COMPOSITION OF PROTEIN MOLECULE 13 



dissolved copper by hydrogen sulphide. A current of air is then 

 passed through the solution to remove the hydrogen sulphide, and the 

 remainder of the hydrochloric acid is either neutralised with the calcu- 

 lated quantity of soda or is removed by treating with silver, carbonate. 

 The solution on concentration deposits the tyrosine. 



Levene prefers the use of hydrochloric acid to that of sulphuric 

 acid for separating tyrosine on account of the difficulty of completely 

 extracting it from the barium sulphate precipitate and of obtaining it 

 in a state of purity. His procedure is the following: The protein is 

 hydrolysed with concentrated hydrochloric acid ; the solution is con- 

 centrated and saturated with gaseous hydrochloric acid. Glutamic 

 acid hydrochloride separates out. The filtrate and washings from this 

 precipitate are concentrated in vacua to remove the greater part of the 

 hydrochloric acid. The solution is then diluted to 7 litres (for 400 

 grammes protein) and boiled with lead oxide till its reaction is alkaline. 

 The lead oxide is prepared by precipitation with baryta, washed by 

 decantation and preserved in the form of a paste. The precipitate of 

 lead oxychloride is filtered off when the solution has cooled. It 

 retains the resinous matters and a nearly colourless filtrate results. 

 The remainder of the chlorine, which is estimated in an aliquot portion, 

 is removed by means of the calculated quantity of silver sulphate, the 

 excess of lead by adding sulphuric acid and passing in hydrogen 

 sulphide, and of sulphuric acid by baryta. On concentrating the solu- 

 tion to one-seventh almost pure tyrosine separates out, which can be 

 filtered off, washed, dried, arid weighed. A portion of the other amino 

 acids can be obtained by further concentration, and treated for 

 leucine and valine. The diamino acids are then precipitated (p. 11) 

 and the filtrate is treated for the other monoamino acids. 



The estimation of tyrosine is thus effected by weighing the crystals 

 which are obtained by the above procedures. 



A new method of determining the presence of tyrosine by bromi- 

 nation was introduced by Horace Brown and employed by Adrian 

 Brown and Millar in 1906 for estimating the rate at which tyrosine is 

 split off from proteins by the action of trypsin. This method has 

 recently been used for the estimation of tyrosine in proteins by Plim- 

 mer and Eaves. 1 The values for most proteins agree very closely with 

 those obtained by isolating and weighing the tyrosine, but are slightly 

 higher. 



The separation of cystine and tyrosine when they are obtained 



1 As yet unpublished. 



