72 A. Neuberger 



used large animals with blood volumes of about 160-180 ml.; 

 moreover, we only took about 3 ml. of blood for each sample 

 and altogether not more than eight specimens per animal. 

 The haemoglobin content did not show any marked changes 

 during the experiments. 



The interpretation which we adopted at first was that the 

 rabbit red cell has a much shorter life span, mean value about 

 50-60 days, than that of man, and that the scatter of life 

 spans for individual cells is very much greater. However, 

 more recently, we have become doubtful about the validity 

 of our assumption that the persistence of the label in the 

 haem is necessarily a quantitative measure of the life span, 

 i.e. the period between the release of the cell into the circula- 

 tion and the disintegration of its morphological structure. 

 The possibility that haemoglobin in the mature human 

 erythrocyte is metabolically active was already mentioned 

 above and this may apply to an even greater extent to the red 

 cell of the rabbit. Recently Benard, Gajdos and Gajdos- 

 Torock (1950) have injected folic acid and vitamin B^g into 

 normal rabbits and have found marked increases within 48 

 hours in the number of red cells without anv significant 

 change in the haemoglobin content. These results are most 

 readily explained by assuming some redistribution of haem 

 or haemoglobin between old and new cells. It was partly in 

 order to get further information on this point that we carried 

 out experiments in which various labelled precursors of the 

 components of the red cells were administered; the haem, the 

 different amino-acids of the globin and of stromatin, and in 

 addition cholesterol were isolated and their radioactivities 

 determined. 



Cholesterol in the Rabbit Red Cell 



A rabbit was injected with ^^C-carboxyl-labelled acetate 

 and methylene-labelled glycine (Muir, Perrone and Popjak, 

 1951). The latter produced activity in the protoporphyrin 

 and glycine residue in the globin, whilst the former gave rise 

 to activity mainly in the cholesterol and, with the dose used. 



