RADIOACTIVE INDICATOKS IN BIOCHEMISTllY 969 



inorganic P, we may conclude that ^ of the glycerophosphate molecules 

 present in the liver was renewed during two hours^^"^ When carrying out 

 such calculations we must taken into account the change with time in the 

 activity of 1 mgm of inorganic phosphate, usually denoted as its "spe- 

 cific activity". The specific activity of glycerophosphate phosphorus, for 

 example, is the percentage of administered ^sp present in 1 mgm of that 

 phosphorus. Itis the specific activity of the glycerophosphate P measured 

 at the end of the experiment which has to be compared with the mean value 

 of the specific activity of the inorganic P during the experiment. In an 

 experiment lasting two hours the end value and the mean value of the 

 specific activity of the inorganic P in the liver of the rat happen to be 

 about identical. A formula which enables us to calculate for any organ 

 and at any time the rate of turnover from data indicating the change 

 with time of the specific activity of both the precursor and the com- 

 pound considered is given by Zilversmit and his associates'^). 



The greatest difficulty we encounter when determining the renewal 

 rates of molecular body constituents is often the lack of knowledge of 

 the pertinent precursor of the compound. We can probably assume 

 inorganic P to be the pertinent precursor of glycerophosphate P. In the 

 case of lecithin, however, the labelled inorganic P may, before its incor- 

 poration into the lecithin molecules, have to be converted into glycero- 

 phosphate P or another intermediary compound. If the formation of 

 glycerophosphate takes an appreciable time, in the first phase of the 

 experiment, then after administration of labelled inorganic phosphate, 

 the newly formed lecithin molecules do not become labelled as they 

 were synthesized with the participation of yet inactive glycerophosphate 

 molecules. When calculating the renewal rate of glycerophosphate by 

 comparing the specific activities of the liver inorganic P and hver lecithin 

 P we thus underestimate the turnover rate of lecithin. This would not be 

 the case if we compared the specific activities of the glycerophosphate 

 P and the lecithin P of the liver. 



In the liver of the dog, for example, the half-life of the lecithin mole- 

 cules was calculated, inorganic P being assumed to be the pertinent 

 precursor, to be 12.5 hours, while with the assumption that glycero- 

 phosphate P is incorporated into the lecithin molecule, the much lower 

 value of 3.6 hours was obtained(i9>. These figures indicate the half-hfe 

 of the average lecithin molecule of the liver. When, however, investi- 

 gating the renewal rate of lecithin present in different cellular fractions 

 of the liver, pronounced differences are found. As seen in Table 2 the 



^1') Hevesy and Hahn, Kgl. DanskeVidenskab. Selsk. Biol. Medd. 15, No. 5 (1940). 

 ^18) Zilversmit, Entenmann and Fishleb, J . Gen. Physiol. 26, 325 (1943). 

 ^19) Zilversmit, Entenmann and Chaikoff, J. Biol. Chem. 176, 193 (1948); 

 PopjAe and Muir, Biochem, J. 46, 103 (1950). 



