MOLECULAR WEIGHT CLASSES OF RESPIRATORY PIGMENT 311 



relative aflfinity of the pigments for oxygen and carbon monoxide. In the 

 samples of hemoglobin they used, an approximately linear relationship was 

 found between log K {K being the coefficient of partition between oxygen 

 and carbon monoxide) and the "span" or shift in wavelength of the absorption 

 band. Figure 1 shows the relationship between these two quantities in a 

 number of different hemoglobins. Extension of the measurements to myo- 

 hemoglobin by Roche {2321a) and Theorell {2761) showed that the point 

 for myohemoglobin lies in the straight line given in Figure 1. The generality 

 of the relationship has, however, been destroyed by the recent work of Keilin 

 and Wang {1503a) on the hemoglobin of root nodules and of the tracheal 

 cells of Gastrophilus. The latter pigment has a greater affinity for oxygen 

 than for carbon monoxide. 



4. MOLECULAR WEIGHT CLASSES 

 OF RESPIRATORY PIGMENT 



4.1. Classification 



The comprehensive investigations of respiratory pigments carried 

 out with the ultracentrifuge by Svedberg and co-workers {2711,2721) 

 have shown a more complicated differentiation on the basis of molec- 

 ular weight than have spectroscopic observations. In the main, 

 these have been determinations of sedimentation velocity and in 

 only a few cases has the sedimentation equilibrium been measured. 

 This is probably due to the fact that the former requires only one 

 milliliter of solution and can be used when several species of molecules 

 are present in the solution, while the accurate determination of the 

 sedimentation equilibrium requires purification which may be difficult 

 with the invertebrate hemoglobins. 



We will divide the respiratory pigments into two main classes, 

 those which are contained within an erythrocyte and those which 

 are found free in the vascular system. In the latter class are included 

 chlorocruorin and a number of erythrocruorins. This step is further 

 justified by the difference in isoelectric point found between intra- 

 cellular and extracellular invertebrate pigments. 



4.2. Extracellular Oxygen Carriers 



Table III shows the distribution of the extracellular invertebrate 

 oxygen carriers, grouped according to the values found for the sedi- 

 mentation constants. They are compared with the value Svedberg 

 and Hedenius derive {2717,2721) by assuming that the oxygen carriers 

 are composed of a number of subunits containing one heme per 



