Studies on Mammalian Red Cells 



73 



produced only slight activity in the porphyrin. The time 

 curves for radioactivities of the cholesterol and porphyrin 

 respectively were very different (Fig. 2) and results show that 

 the cholesterol of the cell membrane is metabolically active 

 to a much greater extent than the porphyrin. The half-life 

 of red cell cholesterol was found to be about 12-14 days, the 



300 



2 



•|200 

 ♦J 



u 



100 



240 



10 



20 



160 



30 

 TIME (DAYS) 



40 



50 



80 



■r. « T^ J- ^- •^- / counts X molecular weight \ . 



Fig. 2. Radioactivities I ^^ — I of proto- 



\ 1000 I 



porphyrin and cholesterol digitonide obtained from a rabbit 

 which had received 57 fxc ^U^MCU^.COOU and 100 /iC 

 CHg.^^COOH. Right hand ordinate refers to the radioactivity 

 of protoporphyrin, left hand ordinate refers to that of cholesterol 



digitonide. 



radioactivity changing in an approximately exponential 

 fashion, whereas the activity of the protoporphyrin fell off 

 linearly. It is also of interest that the maximum activity 

 of the cholesterol was reached after 24 hours, whilst that of the 

 porphyrin was obtained after seven days. Incubation of 

 normal rabbit red cells at 37 °C. with labelled acetate showed 

 that these cells can synthesize cholesterol in vitro. It would 



