80 A. Neuberger 



the dilution of glycine probably being slightly larger. This 

 fact seems at first surprising since glycine, unlike valine, is 

 synthesized by the animal. It does therefore follow that 

 the dilution of labelled glycine is largely due to the rapid rate 

 of protein turnover leading to the liberation of both essential 

 and non-essential amino-acids. 



Stroma Proteins 



We have also isolated glycine from the washed ghosts of 

 the red cells. The insoluble fraction thus obtained is a mix- 

 ture which still retains some haemoglobin. Results (Table I 

 and Fig. 4) show that there is no clear correlation between the 

 radioactivity of the glycine in this fraction and that of the 

 glycine in haemoglobin. The relatively low count at 12 hours 

 suggests that at least some of the stroma protein is formed at 

 an early stage in erythropoiesis. 



Summary 



1. Measurements of the life span of the human red cell by 

 the agglutination method of Ashby and by the isotopic 

 labelling of haemoglobin have been compared. Certain 

 apparent discrepancies are discussed and the possibility is 

 considered that the haemoglobin in the mature human red 

 cell may not be completely inert, or that there is transfer of 

 haem or haemoglobin between circulating cells of different ages. 



2. Experiments reported earlier on the life-span of the 

 rabbit red cell indicate that these erythrocytes have normally 

 a shorter life span of 50 days. As an alternative the possibility 

 is considered that the haemoglobin of the rabbit red cell is 

 particularly labile and that the isotopic labelling of the haemo- 

 globin does not provide a valid method of measuring the life 

 span of the cell. 



3. Experiments are discussed which show that the choles- 

 terol of the rabbit red cell is metabolically active, with a 

 half-life of 12-14 days, and can be synthesized in vitro. 



4. It was found that both in the rat and the rabbit the 

 ratio of molar radioactivities of the porphyrin and glycine 



