PROTEIN DIFFERENCES 317 



Horse hemoglobin has been one of the most thoroughly analyzed 

 proteins and, in Table VII, we compare the data in other species with 

 the values found in this protein. 



In view of the paucity of experimenta data, detailed discussion of 

 generic differences is impossible, although a better comparison may 

 now be made with the invertebrate pigments. Their arginine, his- 

 tidine, and lysine contents are seen to be far outside the range of 

 generic variation in the highest phylum. In addition, the different 

 annelid pigments vary much more than do the different vertebrate 

 pigments. At one time Block {291,292) claimed that the ratio 

 iron :histidine: lysine: arginine was 1:3:8:9 in the hemoglobins of 

 horse, ox, sheep, and dog. Subsequent work (c/. Vickery, 2877) has 

 indicated that this ratio can only be considered approximate; the 

 arginine content of horse hemoglobin, for example, is nearer fourteen 

 molecules per mole hemoglobin than the twelve molecules expected 

 on Block's theory. 



Generic differences in the content of the sulfur containing amino 

 acids were jfirst shown by analysis of total sulfur (c/. Valer, 281^5). 

 The earlier values obtained for cysteine must now be viewed with 

 suspicion unless the prosthetic group was removed before hydrolysis, 

 since Theorell {2770) has shown that cysteine may condense into the 

 vinyl side chains of the hematin during this process {cf. Chapter V). 

 The variations in the estimates given in Table VII may be considered 

 partly due to experimental error and partly to the presence within 

 the animal of several hemoglobins of differing amino acid content. 

 The latter is almost certainly the explanation for the significant 

 departure from whole numbers found for the molar content of some 

 amino acids {196,2052). Roche and Mourgue {2321) found that, 

 while the total leucine + alanine + valine varied only from 46 to 52 

 molecules per Svedberg unit in six mammalian species, the content in 

 the individual amino acids showed enormous variation, not only 

 between species, but also between individuals of a given species, 

 particularly the dog. They suggest that valine may, to some extent, 

 be capable of being replaced by leucine and/or alanine. In view of 

 these variations we shall not discuss the hypotheses of protein struc- 

 ture based on the frequency of occurrence of amino acids {378,4-33, 

 433a,461, 2052, 2070). 



5.2.3. Myohemoglobin. The myohemoglobins have not been investigated 

 as completely as the hemoglobins. Both class differences and generic differ- 

 ences are present. Thus Rossi {23^6) finds that the iron:arginine:histidine: 



