Protein Specificity 



109 



Permeability of Erythrocytes. Another approach to the chemical differences 

 among species is found in the permeability of red blood cells. The permeability 

 of red cells to various organic molecules has been studied by measuring the 

 swelling and ultimate hemolysis in an isotonic solution of the material to be 

 tested. Table 18 shows the permeability of different mammalian erythrocytes 

 for ethylene glycol, glycerol, and erythritol. This table shows that' there are 

 marked species differences in permeability to these substances and that the 



TABLE 18. PERMEABILITY OF RED BLOOD CORPUSCLES^ 



Time required for 75 per cent hemolysis in various media, 



in seconds unless otherwise stated. 



permeability to water, as indicated by swelling of cells from different species 

 in hypotonic NaCl, is unrelated to the permeability to the organic substances. 

 These striking species differences in permeability reflect differences in the 

 composition of the plasma membrane of the erythrocyte. 



CONCLUSIONS 



The proteins of an individual animal present a vast array. They can be 

 separated by chemical and physical means as fibrous and globular, primarily 

 structural and metabolic. The globular proteins, particularly those of blood, 

 have been separated according to their electrophoretic mobilities. The proteins 

 of one electrophoretic class, for example the hemocyanins or hemoglobins, exist 

 in different molecular sizes as shown by sedimentation in a high-speed centri- 

 fuge; they may also differ in crystal form. 



The most sensitive tests available for differences among proteins of similar 

 function and occurrence are immunological. Antibody-antigen reactions prob- 

 ably have the same sort of basis in molecular structure as has enzyme specificity. 

 Antibodies are built up in response to the antigens of a particular species, and 

 will react with those antigenic proteins (by agglutination or precipitation) but 

 not with the proteins of species distantly related to the original one from which 

 the antigen was obtained. One basis of taxonomy may, therefore, be stereo- 

 chemical. It is possible that equally important specific differences exist among 

 polymerized nucleic acids as among proteins. Antigenic specificity of serum 

 proteins represents one set of characters which can be used along with other 

 chemical and morphological characters to study relationships among animals. 

 Relations within animal classes and phyla have been corroborated, as have the 

 relations of echinoderms to prochordates, and of acanthocephalans to flatworms. 



