312 VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



equivalent weight of approximately 17,000. This point is discussed 

 further in Section 4.4. 



TABLE III 



Extracellular Oxygen Carriers 



" Ec = erythrocruorin, Ch = chlorocruorin. 



^ Actual species investigated may be found in reference 2721, p. 360. 



^ Mol. wt. inserted when diffusion constant or sedimentation equilibrium was measured. 



4.3. Intracellular Oxygen Carriers 



In Table IV the data from vertebrates and invertebrates are 

 combined. As in the case of the extracellular respiratory pigments, 

 we compare the weight distribution with that calculated by assuming 

 that each pigment is composed of a number of subunits. 



4.4. Dissociation of Erythrocruorins 



Svedberg and Erikson-Quensel {271J!f, cf. 2711) showed that certain of the 

 erythrocruorins of high molecular weight dissociated into smaller particles 

 outside certain pH limits. Between pH 0.7 and 2.0 three components of 

 the erythrocruorin of Planorhis are found with molecular weights which are 

 one-half, one-quarter, and one-sixth of that found between pH 2 and pH 8 

 {2721, p. 361). 



The intracellular, low molecular weight pigment from Petromyzon, one of 

 the Cyclostomata, behaves differently. Between pH 4 and pH 10 the sedimen- 

 tation constant is 1.87 X 10"" while outside these limits it commences to 

 aggregate to larger particles. 



4.5. Physiological Implication of Particle Size 



There is a very striking difference between the range of molecular weights 

 found among the intracellular and extracellular pigments. The ability of 

 the extracellular invertebrate pigments to form large aggregates has been 

 considered an adaptation which increases oxygen capacity without at the 

 same time bringing in its train an undesirably high osmotic pressure in the 

 plasma {cf., however, Roche and Chouaiech, 2310). The evolution of the 



