APPLICATIONS OF ISOTOPIC INDICATORS 947 



RATE OF FORMATION 



The rate of formation of labelled organic phosphorus compounds 

 differs much for various compounds and varies greatly willi the organ 

 in which they are located. The labile P of adenosine trij)hosphate, for 

 example, is renewed at a very remarkable rate^^''' ^''* , the second P 

 atom being renewed somewhat more slowly than the terminal atom^^^^ 

 Hexose monophosphate, present in the red corpuscles, was found to 

 be largely renewed within a few minutes /^"^ The formation of labelled 

 phosphatides takes place in the liver and in the intestinal mucosa of 

 the rabbit at a much more rapid rate than in the brain and more quickly 

 than in any other organ^^"\ Desoxyribose nucleic acid, on the other 

 hand, shows a behaviour opposite to that of the phosphatides. The 

 extent of its formation in the liver of adult rats is very low^*^"' "'> ^''^^ 

 amounting to only about 0.1 per cent in the course of 2 hours. In the 

 spleen and in the intestinal mucosa, the renewal of the desoxyribose nuc- 

 leic acid is 20 and 30 times, respectively, more rapid than in the liver. 

 High figures for the rate of formation were found in rapidly growing tissue. 



X-rays were found to obstruct the formation of labelled desoxyribose 

 nucleic a.cid molecules^*'^\ During irradiation with X-rays for 2 hours, 

 applying a dose of 15 r/min, the formation of new (labelled) nucleic 

 acid molecules in Jensen's sarcoma was found to be reduced to about 

 14 of its normal value. In the study of the reduction in labelled nucleic 

 acid formation under the action of X-rays, a new line of attack was 

 opened in the study of the action of such radiation on cell division. 



We calculate the extent of renewal of the compound in question, for 

 example creatine phosphoric acid, by comparing the specific activity 

 of the creatine phosphorus at the end of the experiment with the average 

 specific activity of the free phosphorus present in the tissues cell during 

 the experiment. This calculation is based on the assumption that the 

 labelled free phosphate, or the phosphate of a donor whose P enters 

 rapidly into exchange equilibrium with the free P present in the cells, 

 is incorporated in the creatine phosphate molecule present in the cells 

 of the organ investigated. In the case of phosphatides, the possibility 

 cannot be excluded^*^' *^^ that a precursor of the phosphatides 

 molecule is formed at a comparatively slow rate. In such a case, the 

 calculation of the extent of the renewal of the phosphatide molecules 

 would necessitate knowledge of the specific activity of the precursor 

 P. The renewal figures obtained must therefore be interpreted with 

 caution. Another more pertinent reason for the cautious interpretation 

 of the results obtained is that the molecules of some organic compounds 

 may possil)ly be built up within the phase boundary, where the specific 

 activity of the free P may appreciably differ from that of the intra- 

 cellular free P. 



GO* 



