952 ADVENTURES IN RADIOISOTOPE RESEARCH 



DYNAMIC STATE OF BODY CONSTITUENTS 



The most remarkable result obtained in the study of the application 

 of isotopic indicators is perhaps the discovery of the dynamic state of 

 the body constituents. The molecules building up the plant or animal 

 organism are incessantly renewed. In the course of this renewal, not 

 only the atoms and molecules taken up with the food participate, but 

 atoms and molecules located in one organ or in one type of molecule 

 will soon be found in another organ or in another type of molecule 

 present in the same or in another organ. A phosphate radical taken up 

 with the food may first participate in the phosphorylation of glucose 

 in the intestinal mucosa, soon afterwards pass into the circulation as 

 free phosphate, enter a red corpuscle, become incorporated with an 

 adenosine triphosphoric acid molecule, participate in a glycolytic process 

 going on in the corpuscle, return to the circulation, penetrate into the 

 liver cells, participate in the formation of a phosphatide molecule, after 

 a short interval enter the circulation in this form, penetrate into the 

 spleen, and leave this organ after some time as a constituent of a lympho- 

 cyte. We may meet the phosphate radical again as a constituent of the 

 plasma, from which it may find its way into the skeleton. Being incor- 

 porated in the uppermost molecular layer or into labile apatite of the 

 skeleton, it will have a good chance to escape, but it may also have the 

 good fortune to find a more or less lasting abode in the skeleton. This 

 will be the case when it becomes embedded into protected, non-repla- 

 ceable bone material. 



There are indications that, in the growing organism, the rate of new 

 formation of the molecules is still greater than in a fully grown organism. 

 It was found, for example, by making use of heavy nitrogen, kindly 

 put at our disposal by Professor Urey, as an indicator, that in "old" 

 leaves of the sunflower, which did not develop further during the experi- 

 ment, 12 per cent of the protein molecules present were renewed within 

 12 days. In growing leaves, the replacement of old protein molecules 

 was found to take place at a much higher rate^^^^ 



ScHOENHEiMER and RiTTENBERG^^^) havc showu, by applying labelled 

 nitrogen, that the peptide linkages in the proteins of the animal tissue 

 are opened and reclosed with great ease. They found that the protein 

 molecules in the living body continually change and renew their struc- 

 tures. This discovery is one of the most surprising and outstanding 

 results arrived at with isotopic indicators. 



