320 VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



hemoglobin were analyzed graphically. Brinkman and Jonxis (336) 

 showed that the difference between the fetal and adult pigments 

 could also be detected by measuring the velocity with which mono- 

 layers of protein were formed at an air-water interface. This method 

 is also applicable to mixtures. 



6.1.3. Amino Acid Composition. The only work on differences 

 in amino acid composition between fetal and adult animals is that of 

 Vickery {2880) for the ox. The fetal pigment contained 6.43 ± 

 0.04% histidine, and the adult 6.1 dz 0.05% histidine, differences 

 outside the experimental error. The content of histidine in the fetal 

 and adult hemoglobin corresponds to 27.7 and 29.3 moles of histidine, 

 respectively, per 66,700 g. hemoglobin. 



6.1.4. Size and Shape. In view of the fact that myohemoglobin 

 has a smaller molecular weight and greater oxygen affinity than 

 hemoglobin, the possibility was considered that fetal hemoglobin 

 may have a smaller molecular weight than the adult pigment. 

 Measurement of the osmotic pressure of fetal hemoglobin of the 

 sheep by McCarthy {1800), and of the human by McCarthy 

 and Popjak {1803), gave values identical with those found in the 

 adult pigment. Andersch and co-workers {50) investigated the differ- 

 ence between adult and fetal hemoglobin in the human species by 

 other methods. Electrophoretic patterns were obtained which showed 

 that the fetal pigment had a greater mobility than that of the adult. 

 The sedimentation constant, S20 X 10'^, of adult hemoglobin was 

 found to be 4.73, while for the hemoglobin from a five-day-old and a 

 nine-day-old infant, values of 2.5 and 2.9 were found. Since it is 

 unusual although not unknown (c/. 2310,2313,2721), for the results 

 with the ultracentrifuge to differ from those obtained by measurement 

 of osmotic pressure, their results indicate that the fetal hemoglobin 

 of the human may have the same molecular weight but a less asym- 

 metric molecule. Measurement of the diffusion constant should 

 confirm this. 



That such differences in shape or in stability to dissociation might 

 be present in other species is suggested by evidence from another 

 direction. Wyman and co-workers {3138) measured the solubility of 

 the fetal and adult carboxyhemoglobin of the cow. In strong phos- 

 phate buffers, pH 6.8, the fetal pigment was more than six times as 

 soluble as that of the adult. 



It is evident that the physical chemistry of the fetal pigment 



