RADIOACTIVE IXDICATORS IN BIOCHEMISTRY 971 



heavy water, the heavywater content of the urine was determined; from 

 these and other excretion figures the average life-time of water mole- 

 cules in a test person was found to be 14 days. This value clearly 

 depends not only on the total water content but also on the water 

 intake of the subject. 



In the same investigation the total water content was calculated from 

 the heavy water content of the water drunk and that of the body water 

 after exchange equilibrium between the water molecules taken in and 

 those already present in the organism was reached. The heavy w^ater 

 content of the body water was obtained by determining the heavy water 

 content of urine water. The total water content of the test person was 

 found to form 64% of the body weight. For abnormally adipose indivi- 

 duals figures as low as 40%, for lean ones as high as 70%, have recently 

 been obtainecP^). 



The interest of the biochemist in the life-time of the molecules building 

 up the organism was soon much enhanced by the classical work of 

 ScHOENHEiMER and RiTTENBERG^^*\ who determined first the lifetime 

 of fat molecules and later many other types of molecules in the animal 

 body and, moreover, by the investigation of radio-phosphorus in similar 

 investigations, the first of which was the renewal rate of the mineral 

 constituents of the bone.^""^^ 



Place of Formation of Molecular Constituents 



Having discussed the rate of renew al of phosphatide molecules, I want 

 now to say a few words about the place of formation of phosphatide 

 molecules in the yolk of the hen's egg and in the blood plasma. The 

 determination of the place of formation of body constituents is another 

 important field of application for isotopic indicators. 



Only a few hours after administration of labelled phosphate to the 

 hen, the presence of radioactive lecithin and other phosphatides can 

 be detected in the yolks of the ovary. These phosphatide molecules may 

 have been synthesized in the yolk or carried into the yolk by the circu- 

 lating plasma, an alternative explanation being the incorporation into 

 the yolk of phosphatide molecules built up in the ovary. Now, it can 

 be shown by experiments in vitro that penetration of labelled phosphates 

 into the egg followed by intrusion into the yolk does not lead to the 

 formation of labelled phosphatide molecules^^^^ 



<") Drabkin, Fed. Proc. 9, 182 (1950). 



^"'') J. Biol. Chem. Ill, 175 (1935); Schoenheimer, The Dynamic State of 



Body Constituents, Cambridge, Mass. (1942). 

 <^^)Chievitz and Hevesy, Nature, Lond. 136, 753 (1935). 

 <^®) Hevesy and Hahn, Kgl. Danske Videnskah. Selslc. Biol. Medd. 14, No. 2 



(1938). 



