324 VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



species than of others. A dialyzable, heat-labile substance was shown to be 

 liberated from the cells on hemolysis which shifted the dissociation curve of 

 the diluted hemoglobin back toward that found in the cell. The reaction 

 takes place at pH 7.4, while at pH 9.2 no change in the affinity was observed. 

 The authors excluded glutathione and bicarbonate as being responsible for 

 the effect but did not study the substance further. Its properties seem to 

 exclude both stromatin and cephalin (cf. Chapter VI, Section 4.3.5.). 



Altschul, Sidwell, and Hogness (^5) had observed that treatment with 

 alumina cream increased the affinity of hemoglobin for oxygen. Washing 

 the adsorbate with distilled water did not remove any substance decreasing 

 the oxygen affinity. Addition of the neutral phosphate eluate to carefully 

 purified hemoglobin, however, caused decrease in the saturation of some 

 30% at an oxygen pressure of 1 mm. The eluate was shown to contain 

 hemoglobin which was readily denatured, but beyond this it was not investi- 

 gated. Roughton's work on the relative rates of reaction in the erythrocyte 

 and in solution have been discussed in Chapter VI, Section 6.2.7. 



7.3. Myohemoglobin in the Muscle Cell 



The microenvironment of myohemoglobin is much more complicated than 

 that of hemoglobin in view of the much greater variety of substances within 

 the muscle cell. The character of the visible spectrum of myohemoglobin in 

 muscle is similar in character to that found in the purified substance {1279, 

 2220). There is no doubt that myohemoglobin does exhibit a great affinity 

 for oxygen in vivo {1952, 195J^, and Section 10.). The shape of the dissociation 

 curve in vivo, however, has not yet been determined and the possibility of 

 heme-heme interaction cannot be excluded. In view of the dilution of the 

 pigment this would be unlikely if the pigment exists as particles of molecular 

 weight 17,000. Theorell's results (Chapter VI), however, point to the 

 existence of aggregates under some conditions. 



8. BASES OF ADAPTATION 



8.1. Environment of the Oxygen Carriers 



The function of the oxygen-carrying pigments is to take up oxygen 

 from the environment and transfer it to the tissues. The oxygen 

 pressure at which the carrier is in equilibrium with the environmental 

 oxygen is defined as the loading tension, while the oxygen pressure at 

 which the carrier is in equilibrium with the tissues is defined as the 

 unloading tension. These two oxygen tensions are not fixed but 

 normally vary with changes in environment and in the physiological 

 activity of the tissues. For the purposes of characterizing a pigment, 

 two positions on the oxygen dissociation curve are rather arbitrarily 

 selected, corresponding to these pressures. The loading tension is 

 placed at an oxygen saturation of 95%, the unloading tension at 50%. 



