508 XI. HEMOGLOBIN CATABOLISM, I 



hemoglobin is found. There is Httle doubt that the bile pigment found 

 in nature mostly originates from hematin compounds (cf. Chapter X). 

 Choleglobin and biliverdin have been found in situations in which 

 hemoglobin is in the process of destruction, not only in the verte- 

 brates but also in members of the invertebrate phyla (cf. Section 11.). 

 The breakdown of hemoglobin in the leguminous root nodules appears 

 to follow a similar course (Virtanen and Laine, 2891). 



In spite of the almost universal presence of potentially harmful 

 systems, however, hemoglobin is a relatively stable compound in the 

 living animal. The actual mechanism of hemoglobin breakdown is 

 no longer difficult to understand; the problem is, rather, to explain 

 why it does not occur more rapidly. 



The most important factor in the conservation of hemoglobin 

 among vertebrates is probably its inclusion in the red cell, the bio- 

 chemistry and physiology of which we must therefore discuss. 



2. LIFE SPAN OF THE ERYTHROCYTE 



2.1. Introduction 



Since the balance between the formation and destruction of 

 erythrocytes is under physiological control, information as to the 

 normal life must be obtained under conditions which interfere as 

 little as possible with the controlling mechanism. For this reason, 

 measurements of the rate of decrease of the erythrocyte count during 

 abolition of cobalt polycythemia (5^0) are unlikely to give normal 

 results. Furthermore, the clinical condition of the subject is of the 

 greatest importance. It is doubtful, therefore, how much reliance 

 should be placed on the estimate of the lifetime of the red cell, which 

 Eppinger {697) deduced from the excretion of bilirubin in human 

 subjects with bile fistulae. In spite of this and other difficulties, the 

 results for normal breakdown, which have now been obtained by a 

 variety of completely independent methods, show relatively good 

 agreement. The experimental errors, however, are still too great for 

 the results to show how far the life span in one species differs from 

 that in another. There seems to be no reason to assume that differ- 

 ences will not exist, unless in most species a common process limits 

 the life. At the present time one can only speculate, since with the 

 exception of man different methods have generally been used in dif- 

 ferent species. Many reviews are available on this problem {e.g.. 



