Protein Specificity inr 



of various birds and of the hemoglobins of birds and mammals differ by a 

 factor of two at most; this species difference in mobility is much less than the 

 immunological differences among proteins. ■^- 



Sedimentation of Proteins. Another method of separating blood proteins 

 is sedimentation by high-speed centrifugation. Velocity of sedimentation 

 depends on the weight and shape of the protein molecules. Centrifugal separa- 

 tion has been applied particularly well to the respiratory pigment proteins. "»- 

 The chemistry of these pigments is discussed in Chapter 9. From the sedi- 

 mentation data it is possible to calculate the molecular size of large proteins. 

 For example, the hemoglobin molecule is similar in size in mammals, birds, 

 and fishes (m.w. about 68,000), is slightly larger in amphibians and reptiles, 

 and is less than half as large in cyclostomes (m.w. about 20,000) (see Table 

 51 in Chapter 9). Among invertebrate animals the hemoglobin molecules 

 which are free in the plasma are large (m.w. > 900,000), whereas those in 

 corpuscles are small. The copper-containing pigment hemocyanin always 

 occurs free in the plasma, and its molecular size varies greatly among different 

 species. In general, the molecules of molluscan hemocyanins are very large 

 (m.w. > 2,000,000). Those in arthropods are smaller and often separate into 

 several layers in the centrifuge, each heavier one corresponding to a multiple 

 of a smaller unit (Table 52, in Chapter 9). The blood pigments of inverte- 

 brates, both hemoglobins and hemocyanins, show greater specific differences 

 in size than do vertebrate hemoglobins. 



Serum proteins of some mammals, particularly of man, have been studied 

 by sedimentation and by a variety of other physical techniques.^"- ^^' ^^ 

 These globular proteins are shown to be elongate, all being similar in width 

 but differing greatly in length (Table 17). 



Crystal Structure: Hemoglobins. Hemoglobins from different species have 

 been separated on the basis of the characteristics of their crystals as obtained 

 by evaporation after lysis by ether. It has been claimed ^*^ that the crystals 

 of closely related species are more nearly alike than those of distantly related 

 species. Some exceptions to classical taxonomy are noted. The hemoglobin 

 crystals of the sea lion are similar to those of the bears, and the hemoglobin 

 crystals of seals resemble those of the otter; those of the guinea hen show 

 more resemblance to those of the ostrich than to those of other members of the 



Gallinae. However, the form of the hemoglobin crystals depends on such 

 factors as the amount of salt present when crystallization occurs, and one 

 species may show one or several crystallographic types. Hence specific differ- 

 ences in hemoglobin crystals have not been much used by systematists in 

 recent years. Specific differences in hemoglobins are also shown by lack of 

 addition of solubilities of the pigment from unrelated animals. •" 



Immunological Specificity of Proteins. Far more sensitive than preceding 

 physical methods as indication of differences among serum' and tissue proteins 



