THE INDIVIDUAL GROUPS OF PROTEINS . 449 



THE COAGULABLE PROTEINS OF OTHER ANIMAL 



TISSUES. 



Most tissues on treatment with salt solution yield an extract which con- 

 tains proteins similar to those of muscle ; further, a protein like paramyo- 

 sinogen can be extracted with 10-15 P er cent - ammonium sulphate solution. 

 The bulk of the protein, as in the case of muscle, consists of coagulated pro- 

 tein, like myosin. 



These proteins have been very little investigated except those in the thyroid 

 and crystalline lens. 



Thyreoglobulin. 



Thyreoglobulin is a globulin of the thyroid gland which contains iodine 

 in organic ! combination and to which the physiological action of the gland 

 is said to be due. 



The finely minced gland is extracted with water ; the extract is mixed 

 with an equal volume of saturated ammonium sulphate solution and the 

 precipitate is filtered off and washed with half-saturated ammonium sulphate. 

 It is dissolved in water and again precipitated. The precipitate is dissolved in 

 water and thrown down, either by acidifying and washing the precipitate with 

 acidulated water till free from salt, or by dialysis, or by alcohol precipitation. 



Thyreoglobulin forms a colourless powder, insoluble in water but 

 soluble in salt solutions ; it coagulates at 65-67 in 10 per cent, salt solution. 

 The preparation contains various amounts of iodine, sometimes as much as 

 5 per cent., sometimes none. The iodine is split off by boiling with concen- 

 trated hydrochloric acid ; on boiling with dilute acids iodothyrin is formed 

 which contains up to 14 per cent, of iodine. 



Crystallin. 



The crystalline lens of animals contains apparently two globulins which 

 have been termed a- and /?-crystallin. 



a-crystallin. The lens is treated with water and the filtered solution is 

 carefully acidified with acetic acid. The precipitate so formed is dissolved 

 in dilute ammonia and re-precipitated by acid. 



fi-crystallin. The inner quarter of the lens is treated with water and 

 the filtered solution precipitated with acetic acid. The filtrate is neutralised 

 and saturated with magnesium sulphate. The precipitate is purified by 

 solution and re-precipitation and the final solution is dialysed or precipitated 

 by alcohol. 



a-crystallin is precipitated by acetic acid or by carbon dioxide from 

 neutral solution. It coagulates in aqueous solution at 72. a-crystallin is not 

 a typical globulin, but resembles pseudo-globulin and also the nucleoprotems. 



/?-crystallin in salt solution coagulates at 63. /3-crystallin is a typical 

 globulin. 



