TURNOVER OF LECITHIX. CEPHALIX AND SPHINGOMYELIN 



320 



contradiction to that obtained by using labelled phosphate or elaidic 

 acid as indicators. Phosphatides containing iodised fatty acids are 

 possibly selectively taken up by the corpuscles, another explanation 

 being that the molecules of these compounds present in the plasma were 

 decomposed at a faster rate than those incorporated into the stroma. 

 Phosphatides containing iodized fatty acids represent non-physiolo- 

 gical compounds and, as shown by the above example, the results obtai- 

 ned by using such indicators must be interpreted very cautiously. 



Table 14. >— Specific Acttvitv of Phosphatide P and 

 Ac ID Soluble P of the Corpuscles 



<•> The active phosphatide molecules are partly such ones which were taken up from the plasma by a 

 exchange process. 



In this connection, the observation^^) should be also mentioned that 

 in lactating cows during fasting a marked decrease in the concentration 

 of plasma P lipids takes place which persists for several weeks after reali- 

 mentation, but there is no significant change in the amount of red cell phos- 

 phatides. This result also shows the absence of an intense interaction bet- 

 ween plasma phosphatides and phosphatides present in the corpuscles. 



PART II 



Investigation of lecithin, cephalin, and sphingomyelin 



We discussed above the rate of renewal of the average petrol-ether 

 soluble phosphatide molecules; in the following, we wish to describe 

 some experiments in which lecithin, cephalin, and sphingomyelin were 

 separately investigated and their turnover rate determined. Chemically^ 

 cephalin differs from lecithin by containing aminoethanol instead of 

 choline. The biological consequence of this replacement is very signifi- 

 cant(2). Cephalin is highly active in accelerating blood clotting, whereas 



(i)j. A. Smith, Biochem. J. 32, 1856 (1938). 



(2> Comp. E. Chajigaff, M. Ziff and B. M. Hogg, J. Biol. Chem. 131, 35 (1939).. 



