70 A. Neuberger 



to labelling of red cells formed about 60 to 100 days after 

 the beginning of the experiment. Such an assumption, which 

 implies that the average isotope content of the glycine which 

 is utilized for porphyrin synthesis has only decreased after 

 80 days to about 15 per cent of its maximum value, is in 

 conflict with the mathematical treatment of Shemin and 

 Rittenberg (19466) and is altogether unlikely. Moreover, if 

 labelled haem should still be incorporated into newly formed 

 cells 50-80 days after the administration of the labelled 

 glycine, the isotope content of the hsem should rise consider- 

 ably between the 50th and 80th day. This is not so. We 

 suggest therefore that a proportion of the hsem or of the 

 haemoglobin of decaying red cells may be utilized in the pro- 

 duction of new cells even in normal man. It is uncertain 

 whether such a re-utilization, which will normally affect only 

 about 15-20 per cent of the total erythrocytes, involves the 

 whole haemoglobin molecule or only the haem. 



Another, possibly less real, discrepancy concerns the period 

 between the 30th and 100th day. The results of London et al. 

 (1949) and also our own data suggest that there is, at least 

 in some apparently normal subjects, a decrease of about 

 10-12 per cent of isotope content in the haem between the 

 40th and 80th day. Moreover this rate of fall increases 

 greatly after 90 days. On the other hand the results obtained 

 by the agglutination method suggest that hardly any normal 

 cells die before the 90th day. It will be necessary to evaluate 

 critically the limits of error of the two methods and also the 

 mathematical analysis employed in the interpretation of the 

 results before any decision can be reached as to whether this 

 discrepancy is real. At the present time the possibility that 

 the haemoglobin in the normal and mature human red cell is 

 metabolically somewhat active cannot be excluded. 



The Life Span of the Red Cell in the Rabbit 



In this laboratory we have been particularly interested in 

 the red cell of the rabbit (Neuberger and Niven, 1951); for 

 labelling the cells we have been using both ^^C and ^^N (Muir, 



