978 ADVENTURES IN RADIOISOTOPE RESEARCH 



to an extent similar to that of '^'^P incorporation into deoxyribonucleic 

 acid in the above-mentioned, experiments. Furthermore, experiments 

 carried out in Chicago*^^^'''^ brought out a depressing effect of irradiation 

 on the incorporation of ^^C into the purines of ribonucleic acid similar 

 to that observed in the case of the deoxyribo-compound. The incorpora- 

 tion of ^^p into ribonucleic acid^^^^ was found, however, to be markedly 

 less susceptible to the effect of irradiation than its incorporation into 

 the deoxy-compound. 



RENEWAL OF RIBONUCLEIC ACID 



A very appreciable part of the incorporation of ^^p into ribonucleic 

 acid molecules may be due to rephosphorylation of such molecules. 

 If, in contrast to the synthesis of the total molecule the rephosphoryla- 

 tion process is not radiosensitive, we may find an explanation in the 

 difference of the effect of radiation on the incorporation of ^^p and ^^C 

 into the ribonucleic acid molecules. This explanation is very hypothetical, 

 and I mention it mainly because it offers a welcome opportunity to 

 emphasize the fact that isotopic tracers may indicate fundamentally 

 different processes, such as renewal of one of the molecular constituents 

 only or synthesis of the whole molecule from primitive precursors and, 

 furthermore, intermediates between these two extreme cases. 



Let us consider a compound such as adenosine triphosphoric acid. 

 This molecule contains two labile, easily removed phosphate groups, 

 and a third one incorporated into the adenylic acid moiety. In the animal 

 organism, intrusion of the administered labelled inorganic phosphate into 

 tissue cells is followed b}^ a strikingly rapid interchange between the 

 labelled phosphate ions and the labile phosphate groups of the adenosine 

 triphosphoric acid. An interchange may even take place during the 

 passage of the labelled inorganic phosphate through the boundary of 

 the intra- and extra-cellular spaces, as pointed out by Sachs^^^\ The 

 adenylic acid moiety is not involved in this very rapid renewal process. 

 The rate of incorporation of ^sp into the adenosine triphosphate is thus 

 a measure of the rate of rephosphorylation only of that molecule and 

 not of its formation from early precursors. Even in a rapidly growing 

 mammalian organ, in which the adenosine triphosphoric acid content 

 may increase by as much as 1% per hour, such an increase involving 

 1% additional formation of adenosine triphosphoric acid from early 

 precursors, the incorporation of ^^p is negligible compared to incorporation 

 of 32p by rephosphorylation. 



(4ia) Personal communication by Dr. Guzman Barron. 

 (*2) Cold Spring Harbor Sijmp. Quant. Biol. 13, 180 (1948). 



