314 



VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



erythrocyte, with its high specific permeabiHty, enables the development of 

 high oxygen capacities without interfering with the osmotic pressure within 

 the circulation. Parallel with this, we see the disappearance of respiratory 

 proteins of large molecular weight. 



5. PROTEIN DIFFERENCES 



5.1. Isoelectric Points 



This is the only other property investigated as systematically as the 

 molecular weight distribution and the type of prosthetic group {2708, cf. 

 also references in 2721). The division of the respiratory pigments in Table 

 V on the basis of their isoelectric point corresponds roughly with the division 

 into high and low molecular weight classes (2721). 



TABLE V 



Isoelectric Points" 



Site of pigment 



Phylum* 



No. of 

 Hiifner units 



Pigment 



Isoelectric 

 point 



" With the exception of the isoelectric point of chlorocruorin (2S01) the values are 



taken from those tabulated by Svedberg and Pedersen (2721, pp. 357, 358, 360). 

 * Number of species examined given in parentheses. 



It can be seen that the isoelectric points of the extracellular erythrocruorins 

 so far examined lie in the range 4.56 to 5.4. The intracellular erythrocruorins 

 fall in a somewhat higher range, 5.6 to 6.0, while, with the exception of one 

 species, the isoelectric points of the carbon monoxide hemoglobins are in the 

 region of 6.4 to 7.5. The distribution of isoelectric points found is not of 

 much interest by itself, but corresponds to the phylogenetic differences found 

 in amino acid composition. 



