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ADVENTURES IN RADIOISOTOPE RESEARCH 



rate of renewal of the phosphatide molecules present in the cell nuclei, 

 and to a minor extent also those present in the mitochondria, is markedly 

 lower than the corresponding figure for the average tissue phosphatides. 

 The figures were obtained two hours after injection of labelled phos- 

 phate. (20) 



Table 2. — Renewal Rate of Phosphatide P of the Liver 

 Fractions of the Rat 



Ample evidence is available to support the assumption that the incor- 

 poration of labelled phosphate into phosphatide or other organic phos- 

 phorus compounds involves enzymic processes, no "physical" inter- 

 change taking place. In the study of phosphatides in surviving liver 

 slices incubated in a Ringer solution containing labelled phosphate, for 

 example, the formation of "marked" phosphatides takes place under 

 aerobic conditions only^^^^ Absence of oxygen or presence of respiratory 

 inhibitors should clearly not interfere with a non-enzymic type of inter- 

 change. 



It was the introduction of the method of isotopic indicators which has 

 drawn the attention to the notion of the lifetime of the molecules building 

 up the organism, a magnitude formerly not considered. The first lifetime 

 determination, performed almost immediately after the discovery of 

 deuterium, was that of the water molecules of the goldfish^^'^^ The water 

 molecules present in the living goldfish were found to interchange rapidly 

 with those of the surrounding water, and some of the hydrogen atoms 

 (the "labile" hydrogen) of the organic tissue components were found to 

 be replaced at a remarkable rate by the hydrogen atoms of the body 

 water. 



Shortly afterw^ards, the life-time of the water molecules present in the 

 human body was measured^^^''^ Dilute heavy water being used as an 

 indicator, following the intake of a known volume of dilute (0.5 molar %) 



(20) DE Elliott and Hevesy, Acta Physiol. Scand. 19, 370 (1950); cf. Ada, 



Biochem, J. 45, 422 (1949). 

 <^2i)Taubog, Chaikoff and Perlman, J. Biol. Chetn. 145, 281 (1942). 

 (22> Hevesy and Hofer, Z. physiol. Chem. 225, 28 (1933). 

 (22a) Idem, Nature 134, 879 (1934). 



