Originally communicated in Exp. Cell Res. 3, lid (1952) 



41. DETERMINATION OF THE RATE OF RENEWAL 

 FROM THE RATE OF DISAPPEARANCE OF LABELLED 



MOLECULES 



George Hevesy 



From the Institute for Research in Organic Chemistry, 

 University of Stockholm 



The rate of renewal of a type of molecules is usually calculated from the 

 rate of incorporation of the labelled atoms of the pertinent precursor 

 in the molecules considered. If we wish, for example, to known the 

 percentage of desoxyribonucleic acid molecules of the rat spleen, which 

 are formed in the course of two hours, we administer labelled sodium 

 phosphate to a rat and two hours later we compare the specific activities 

 of the desoxyribonucleic acid P, and inorganic P extracted from the 

 .spleen. If the ratio of these specific activities is found to be 0.02, the 

 specific activity of the inorganic P remained constant during the experi- 

 ment, and this P can be considered to be the pertinent precursor of 

 desoxyribonucleic acid P, we can conclude that in the tumour 2 per cent 

 of the desoxyribonucleic acid molecules are present which were formed 

 during the experiment or, more correctly, that at least 2 per cent of 

 these molecules were formed in the course of two hours. If it takes some- 

 time that the labelled precursor reaches the site of desoxyribonucleic 

 acid syntesis the first phase of the synthesis of this compound will not 

 be indicated by the tracer and we shall correspondingly underestimate 

 its rate of synthesis formation. 



If w^e protract the experiment, the specific activity of the inorganic 

 !•* of the spleen decreases more and more and this decrease is followed 

 by a decrease in the specific activity of desoxyribonucleic acid P. We can 

 also calculate the rate of renewal by comparing the specific activities of 

 desoxyribonucleic acid P and inorganic P in this declining activity phase 

 of the experiment. 



The rate of loss of ^"^P by desoxyribonucleic acid molecules in the late 

 phase of the experiment is independent of the precursor problem, but 

 parallel with a loss ^^p by strongly active "old" desoxyribonucleic acid 

 molecules the formation of less active "new" molecules takes place 

 for example in the spleen and the rate of incorporation of ^-P in these 

 molecules is partly determined by the specific activity of the pertinent 

 precursor. Thus by replacing the calculation of the renewal rate from 



27 Heresy 



