Studies on Mammalian Red Cells 69 



Shemin and Rittenberg (1946a) found that glycine was a 

 specific precursor of the nitrogen of haem. They then studied 

 the change of isotope concentration with time of the haemo- 

 globin haem of a subject fed ^^N-labelled glycine (Shemin and 

 Rittenberg, 1946^). There is broad agreement between 

 results obtained by the three methods, indicating that the 

 average life span of the human red cell is about 120 days. 

 This conclusion is also supported by the results obtained on 

 the changes of concentration of ^^N in stercobilin after feeding 

 isotopic glycine to a human subject (London, West, Shemin 

 and Rittenberg, 1950; Gray, Neuberger and Sneath, 1950). 



However, if the data are examined in detail, certain dis- 

 crepancies appear between results obtained by the two 

 methods, i.e. the isotopic labelling of the porphyrin and the 

 agglutination technique. In the Ashby technique the trans- 

 fused cells constitute a mixed population with respect to age, 

 and the mean life expectation is not immediately apparent 

 from the observed data. However, analysis of the survival 

 curves (Callender, Powell and Witts, 1945) indicates (a) that 

 practically no cells survive beyond the 130th day, and (b) 

 that very few cells die before the 90th day. In other words 

 the scatter round the mean value is very small. The main 

 complication in the interpretation of the isotope experiments 

 is the fact that the glycine of the body which is utilized for 

 porphyrin formation will contain significant amounts of ^^N 

 for some time after feeding of the labelled compound; thus 

 the labelled cells are not released into the circulation at one 

 instant, but over a certain period of time. Shemin and 

 Rittenberg (19466) have allowed for this fact in their analysis, 

 and it appears that the release of labelled cells is practically 

 completed by the 20th day. However, the data of the 

 Columbia workers (Shemin and Rittenberg, 19466, London, 

 Shemin, West and Rittenberg, 1949) and also our own obser- 

 vations indicate that the ^^N content of the porphyrin is still 

 about 20 per cent of the maximal value 170 days after feeding 

 and still about 12 to 15 per cent after 220 days. It might 

 be argued that this unexpectedly high isotope content is due 



