334 VII. COMPARATIVE BIOCHEMISTRY OF HEMOGLOBINS 



air in mammals. At partial pressures of carbon dioxide even lower 

 than that found in alveolar air, the oxygen saturation of these hemo- 

 globins may be only of the order of 50% of that found at atmospheric 

 pressure. Mammalian hemoglobin must, therefore, be considered as 

 adapted not only to the temperature of 35-40° C. at atmospheric 

 oxygen pressure, but also to the anatomy of the aerial respiratory 

 system which requires that oxygen loading take place at relatively 

 high partial pressure of carbon dioxide. 



11.3. Role of Absolute Reaction Velocities 



Salomon {21^23) has measured the rates of dissociation of two 

 annelid hemoglobins, while Davenport {533) has reported data on 

 the pigments in Ascaris. The pigment of Glycera, carried in corpus- 

 cles, has the same dissociation rate as has human oxyhemoglobin, 

 both measured in dilute solution by the reaction meter of DuBois 

 (636). The extracellular high molecular weight pigment from Lum- 

 hricus dissociates about one-third as fast. The hemoglobin from the 

 body wall of Ascaris has a dissociation rate 1/2500 and the hemo- 

 globin of the parenteric fluid 1/10,000 of that of mammalian hemo- 

 globin. If we consider the relatively sluggish movements of the 

 annelids in the light of the absolute value for the half dissociation 

 time for the pigment in Lumbricus, 70 milliseconds, it seems reasonable 

 to conclude that, in the free living annelids, the velocity at which 

 the gaseous reactions take place is not the limiting factor in their 

 functional adaptation. In the case of Ascaris, which leads an even 

 more inactive existence, the dissociation rate of the oxygenated 

 carrier is probably still faster than its metabolic processes. 



Myohemoglobin is present in invertebrates. It is found in the 

 pharynx of Limnaeus and Paludina (16^7), in the heart and adductor 

 muscles of other molluscs (125,527,1910). Lankester (1647) pointed 

 out the association between its distribution and the activity of the 

 muscle. T'nfortunately, it is not yet possible to decide whether 

 this invertebrate myohemoglobin can be classed with mammalian 

 myohemoglobin on the grounds of its affinity as distinct from its 

 distribution. Even if its dissociation rate were slower than that of 

 mammalian myohemoglobin, it is doubtful if it would become a 

 limiting factor in the velocity of the heart beat or the contraction of 

 the adductor muscles in the molluscs. Only in the vertebrates does 

 muscle physiology approach the limits set by the velocity of the 

 gaseous reactions of the oxygen carriers. In some species of insects. 



