344 ADVENTURES IN RADIOISOTOPE RESEARCH 



different cephalin fractions obtained by fractional crystallisation of 

 the total cephalin extracted from the organ in question did not show- 

 large variations, it is not probable that the above result can be explained 

 as due to different rates of new formation of cephalins of different chemi- 

 cal composition. 



In fractional crystallisation of alcoholoc cephalin solutions, only 

 minor differences in the specific activity of the fractions were noticed. 

 The least soluble fraction extracted from the liver showed, for example, 

 a turnover rate of 4.47, while the value found for the average fraction 

 was 4.05. When organs were extracted first with ether and then with 

 hot alcohol, the lecithin prepared from the first extract was found to 

 be somewhat more active than was the lecithin prepared from the 

 alcohol extract (see Table 19). 



Since the renewal of cephalin is an enzymatic process, its velocity 

 should be determined by the effectivity of the enzymes present. It is 

 probable that that part of the cephalin which is located in such a region 

 of the cells, where the enzymatic action is very pronounced, is renewed 

 at a very fast rate. It is also probable that this "fast" fraction has a 

 different biological significance from the "slow" fraction. The fact that 

 the phosphatides have a much larger turnover in some organs than in 

 others induced Sinclair^^) to distinguish between metabolic and non- 

 metabolic phosphatides. The former ones found in the liver, for example, 

 should be involved in fat metabolism; the latter ones, found for example, 

 in the muscle, should play an important role in building up cell membra- 

 nes. Our results suggest the interpretation that we have in all the organs 

 nvestigated a "fast" and a "slow" cephalin fraction as well. The "fast" 

 fraction is the smaller one. To what extent the "fast" cephalin and other 

 phosphatide fractions are involved in fat metabolism is under investi- 

 gation. 



Summary 



Labelled sodium phosphate was administered to rabbits, rats, frogs and laying 

 hens. In order to keep the concentration of the labelled phosphate in the plasma 

 constant, labelled phosphate was injected from time to time throughout the 

 experiments. 



The specific activity of the inorganic P extracted from the plasma and the 

 organs was measured at intervals. From these data the average specific activity 

 of the cellular inorganic P prevailing during the experiment was calculated. 



The phosphatides present in various organs were extracted as well, and the 

 specific activity of th-^ f)hosphatide P and also of the lecithin, cephalin and sphin- 

 gomyelin P determinod. 



The knowledge of the average specific activity of the cellular inorganic P 

 during the experiment and that of the phosphatide P at the end of tht experimen^ 



^i^R. G. Sinclair, Physiol. Rev. 14, 357 (1934). 



