PRINCIPLES OF TRACER METHODOLOGY 45 



degradation of two substances or the differential behavior of two parts 

 of a given molecule. Double-isotope studies, by allowing two sets of 

 data to be obtained from the same organism, may save considerable time 

 and expense, especially where the experiment involves systems that are 

 difficult to prepare. This is the situation for animals or plants brought 

 to special nutritional status or raised under special environmental 

 conditions. 



For example, pairs of elements (C'^-P^^^ P^'-K*'-, P^^'^-Cs'") have been 

 simultaneously traced in the plant to give information on the mechanism 

 of translocation (36, 44). In animal nutrition and physiology the close 

 relationships of calcium and phosphorus have made the use of this pair 

 of elements very fruitful. Simultaneous measurements have been made 

 of the endogenous fecal phosphorus and calcium in cattle (103). By the 

 concurrent measurement of sodium and calcium disappearance from the 

 plasma of dogs, it was shown that the transcapillary movement of the two 

 elements was about the same except for the exchange into bone exhibited 

 by the calcium (78). 



The synthesis of nucleic acids has been studied by the simultaneous 

 injection of carbon-labeled glycine and P^'- into rats (104). The specific 

 activities of the P^^ and C^'* in the purine nucleotides from DNA (desoxy- 

 ribosenucleic acid) were similar, indicating that both precursors were 

 incorporated into some intermediate at the same time. However, the 

 specific activity of the P^^ in the RNA (ribosenucleic acid) purine nucleo- 

 tides was considerably higher than that of C'^ indicating that the P^^ was 

 incorporated earlier in this case. 



A very powerful method is available in tho.se instances where it is pos- 

 sible to use two different radioisotopes of the same element (Na-^-Na^'*, 

 Fe"-Fe^^ Sr^^-Sr^", Y^o-Y^S Ag^'O-Ag"'). A considerable advantage Hes 

 in the correction for secondary losses, which is an extension of isotope 

 dilution, as discussed on page 16. For example, assume that radioisotope 

 A is administered to the system and that an unknown amount of it 

 becomes located in a sample. A known amount of radioisotope B is 

 added to the sample, which is then analyzed for both isotopes. It is 

 clear that the following is true: 



C = A^ (1-38) 



where C = amount of radioisotope A in sample 



A = amount of radioisotope A measured in sample or in given por- 

 tion of sample 

 Ba = amount of radioisotope B added to sample 

 Br = amount of radioisotope B measured in same portion of sample 

 in which A is measured 



