968 ADVENTURES IN RADIOISOTOPE RESEARCH 



leads to a molecular interchange only without any change of concent- 

 ration. The same applies to interchange permeability. The individual 

 sodium ions are soon found in the corpuscles and soon in the plasma 

 without disturbing the prevailing concentration differences between the 

 two phases. 



Although by far most of the sodium of the organism is found in the 

 circulating body fluid, yet it is not only a main constituent of that fluid 

 but fulfils many important tasks besides this. This is obvious, inter alia, 

 from the recent results obtained by Buchanan and his colleagues^^*^) 

 in their investigations of glycogen formation in which radio-carbon was 

 applied as indicator. When liver slices were incubated in the presence 

 of labelled pyruvate, formation of labelled glycogen and other labelled 

 carbohydrates was found to take place. In these experiments the addi- 

 tion of sodium, but not however of potassium, to the incubating medium 

 was found to stimulate 2- and 3-folcl the formation of carbohydrates 

 other than glycogen, while addition of potassium promoted the formation 

 of glycogen. 



LABELLING OF MOLECULAR BODY CONSTITUENTS 



Rate of Molecular Renewal 



In the study of the fate of molecular constituents, such as glycero- 

 phosphate, nucleic acid, or haemoglobin, in the organism we frequently 

 do not introduce into the body labelled molecules, but marked precursors 

 of these compounds. Introduction of glycerophosphate containing radio- 

 phosphorus into the circulation, for example, will soon be followed by 

 an enzymatic decomposition of that compound, a small percentage only 

 of the glycerophosphate administered reaching the liver and other 

 organs. It is not the determination of this percentage we are mostly 

 interested in; we wish to know primarily the rate and the main place 

 of formation of glycerophosphate. This information we can obtain by 

 introducing labelled inorganic phosphate, some of which rapidly reaches 

 the liver and is incorporated into newly formed glycerophosphate mole- 

 cules. By comparing the activity of 1 mgm of liver inorganic P with that 

 of 1 mgm of liver glycerophosphate P, we arrive at a figure characterizing 

 the rate of formation or, as the glycerophosphate content of the liver 

 remains constant during the experiment, the rate of renewal of glycero- 

 phosphate molecules or, more correctly, for reasons stated below, the 

 lower limit of that rate. If after the lapse of two hours 1 mgm of glycero- 

 phosphate P has an activity corresponding to ^J of the activity of the 



^i«^ Buchanan, Hastings and Nesbett, J. Biol. Chein. 180, 435, 447 (1949). 



