336 



ADVENTURES IN RADIOISOTOPE RESEARCH 



in experiments of short duration tiie cephalin P extracted from the liver, 

 for example, is found to be more active than the lecithin P, in experi- 

 ments of long duration both fractions are found to have about the same 

 activity. Not only cephalin and lecithin extracted from the organs 

 of the rabbit show this behaviour, but also the phosphatide fractions 

 secured from the organs of rats, frogs, hens, and of isolated cat liver. 



50 Days 

 Fig. 0. Turnover of lecithin and cephalin in the brain. 



That, in the case of the muscle and brain tissue, cephalin is found 

 in experiments of long duration as well to have a faster turnover rate 

 Than lecithin is in no way in contradiction to the conclusion arrived 

 at when investigating the liver fractions. All phosphatide fractions 

 present in the muscle, and especially those in the brain, are renewed 

 at a comparatively slow rate. This remark applies also to the "fast" 

 cephalin fraction present which, though "fast" relative to the average 

 cephalin or lecithin, is in fact "slow". This slowness has the effect that 

 seven days do not suffice to reach the point where the amount of label- 

 led lecithin is larger than that of the labelled cephalin. The considera- 

 tions made above are iUustrated by Figs. 5 and 6. 



Brain phosphatide 



While, in the case of other tissues, the penetration of phosphate from 

 the plasma into the interspaces can be considered as an almost momen- 

 tary process and, accordingly, the specific activity of the plasma inor- 

 ganic P can be taken to be equal to that of the extracellular inorganic P, 

 It cannot in brain tissue. We find that after 4 hours only 1/3 of the amount 

 of labelled phosphorus (inch organic P) is present in the brain tissue, 

 which we should expect to be present in the extracellular space alone 

 in case of a prompt distribution of the labelled phosphate between the 



