490 ADVENTURES IN RADIOISOTOPE RESEARCH 



or alternatively dilfused from the plasma into the corpuscles^^\ As already 

 mentioned, within a day, the plasma phosphatides of a daily laying hen 

 were replaced up to 82%, by phosphatide molecules carried into the 

 circulation from the liver and other organs. In spite of this thorough- 

 going replacement of the plasma phosphatides, the phosphatide mole- 

 cules present in the corpuscles are renewed only to an extent equal to 

 I of that of the plasma phosphatides in the course of 28 hours. In human 

 subjects, after the lapse of one day, the corpuscle phosphatide shows 

 a specific activity amounting to only | or less of that of the plasma 

 phosphatide P and even after the lapse of 8 clays" the ratio is still |. 

 When the corpuscles are formed from plasma containing labelled phos- 

 phatides they are bound to contain such. x\s seen above, even after the 

 lapse of a week the labelled fraction of the plasma phosphatides is still 

 twice as high as that of the phosphatides present in the corpuscles. 

 From this result we can conclude that less than half of the corpuscles 

 are produced in a week. One could object to the above conclusions on 

 the ground that the phosphatides are composed of different constituents, 

 lecithin, cephalin and so on, each of which may contain very different 

 fatty acids and, if those components which are mainly represented in 

 the corpuscles are renewed at a slower rate than those chiefly found in 

 the plasma, this difference may also tend towards a low value for the 

 specific activity of the corpuscle phosphatide P. In human blood, 

 the composition of the corpuscle phosphatides was found to be not 

 very different from that of the plasma phosphatides*^^) ^nd as we have 

 found that, after a lapse of a day, the specific activity of the corpuscle 

 phosphatides is only 8 — 19% of that of the plasma phosphatides (comp. 

 Table 8) we are justified in concluding that at least the most active com- 

 ponent of the phosphatide mixture of the corpuscles must show a lower 

 specific activity than its counterpart in the plasma. 



This fact definitely excludes the possibility that the active phospha- 

 tides are formed in the corpuscles and diffuse into the plasma. The result 

 discussed on page 486, according to which the corpuscle phosphatides 

 contain more iodised fat than the plasma phosphatides, is presumably 

 to be explained by a greater preference of the corpuscles for phosphatides 

 containing an iodised fatty acid component. 



(i) An investigation carried out recently in collaboration with L. Hahn lead 

 to the result that a fairly slow exchange between a part of the phosphatide molecu- 

 les present in the corpuscles and those present in the plasma takes place. In the 

 course of 4 hours about 5% of the phosphatides present in the corpuscles were 

 exchanged. 



(2)E. Kirk, J. Biol. Chem. 123, 637 (1938). According to this experimentor 

 plasma phosphatide contains as an average 13% lecithin, 47% cephalin, and 

 40% sphingomyelin, while the corresponding figures for the corpuscles are 16, 

 60 and 24. 



