38 THE GENERAL CHARACTERS OF THE PROTEINS 



iodine was first detected by Baumann in 1895. The iodine containing 

 protein has been exhaustively studied by Oswald. The amount of 

 iodine varies in different preparations ; Oswald found 0*46 per cent. 

 in the thyro-globulin prepared from pigs' thyroids, O'86 from ox thy- 

 roid, and O'39 from sheep's thyroid ; in human thyroid he found from 

 0*07 to 0*5 1 per cent, (the latter after potassium iodide administration) . 



Halogens have also been found in certain marine animals, es- 

 pecially in the skeletons. The axial system of Gorgonia cavolini 

 contains, according to Drechsel, about 8 per cent, iodine (calculated 

 on the amount of dry substance), which is combined with the protein 

 substance known as gorgonin. Sponges also contain iodised pro- 

 teins according to Hundeshagen and Harnack. The latter by the 

 hydrolysis of sponges with mineral acids isolated the protein body 

 iodospongin containing 9 per cent of iodine. 



Various other proteins with halogen content have been recently 

 isolated from marine organisms by Morner. 



SECTION XV. THE TYROSINE FACTOR OF PROTEINS. 



Each individual protein yields on hydrolysis certain definite quan- 

 tities of amino acids. The method of determining these quantities 

 with any degree of accuracy is unknown for the majority of the 

 amino acids ; even by the esterification method of Emil Fischer only 

 approximate results are obtained. 



There are, however, at least one or two hydrolysis products, of 

 which the yield can be readily determined when only small quantities 

 of the protein from which they are derived is available. The quan- 

 tity of the cystine group can be estimated by a determination of the 

 total sulphur of the molecule 1 (see p. 30). Another hydrolysis pro- 

 duct, the amount of which can be readily determined, is tyrosine ; 

 it is probable that the tyrosine factor (i.e., the percentage of tyrosine 

 liberated on hydrolysis) will form a definite characteristic for each 

 protein. 



There are several earlier investigations on this subject, which are 

 summarised in a paper by Reach. In the researches described in 

 this and the previous papers the tyrosine was obtained in the form 

 of crystals, which were weighed. A more accurate method is, how- 

 ever, due to J. H. Millar, who has shown that tyrosine can be readily 

 estimated in even complex mixtures by titrating a solution of a 

 protein containing hydrochloric acid and potassium bromide with 

 a standard solution of potassium bromate, until a potassium iodide- 

 starch indicator denotes the presence of free bromine. The reaction 

 which takes place may be represented by the following equations : 



NaBrO, + sKBr + 6HC1 = NaCl + sKCl + 3Br 2 + 3 H 2 O. 

 C 6 H 4 . (OH). CH 2 . CH(NH 2 ) . COOH + 4 Br = C 6 H 2 Br 2 . (OH) . CH 2 . CH . (NH 2 ) . COOH. 



According to the theory of these equations 1765 grams of bro- 

 mine would be required for each gram of tyrosine, or about r8'8 c.c. 



- potassium bromate solution. This method has not yet been much 



applied to the investigations of the hydrolysis products of proteins. 

 Adrian Brown and E. T. Millar have, however, recently shown 



1 On the assumption that cystine is the only sulphur-containing group. 



