74 A. Neuberger 



thus seem that part of the cell membrane, i.e. the hpid 

 components, is metabohcally much more active than haemo- 

 globin, even in the circulating erythrocyte. Exchange with 

 the lipids of the plasma has been demonstrated for the red 

 cell phospholipid (Hahn and Hevesy, 1939; Zilversmit, 

 Enteman, Fischler and Chaikoff, 1943). 



Globin and Porphyrin 



It is now well established that eight methylene carbon atoms 

 of glycine are utilized for the formation of protoporphyrin 

 (Muir and Neuberger, 1950; Radin, Rittenberg and Shemin, 

 1950), four for the four methyne carbon atoms and four for 

 one carbon atom in each of the four pyrrolic rings (Wittenberg 

 and Shemin, 1950). Glycine is of course also used in the 

 synthesis of the globin part of the hsemoglobin molecule. If 

 the conversion of glycine into protoporphyrin takes as long 

 as the incorporation of glycine into globin and if both chains 

 of reactions utilize the same intracellular pool of glycine, the 

 specific activity (expressed in molar equivalents) of the proto- 

 porphyrin should be eight times that of the globin glycine. 

 If one component is formed earlier or becomes metabohcally 

 inert sooner, then the activity of that component should at 

 first be lower than that of the other component which is formed 

 later. However, provided the rate of red cell production is 

 constant, the ratio of the molar activities of the protopor- 

 phyrin to globin glycine should, as the activity-time curve 

 flattens out, converge towards eight. 



We have tested these predictions both in the rabbit and in 

 the rat. In the rabbit experiments fairly large blood samples 

 were obtained from the same animal and the absolute values 

 of the activities are therefore somewhat influenced by the 

 anaemia produced. However, this is unlikely to affect the 

 ratios of activities in which we were primarily interested. In 

 the rat experiments crystalline haemoglobin from two animals 

 was used for each time interval. In these experiments too 

 the absolute values are not strictly comparable owing to 



