RADIOACTIVE INDICATORS IN BIOCHEMISTRY 



993 



cycle of blood corpuscles. The rapid decrease with time in the specific 

 activity of the precursor proves also to be useful in the study of the 

 change of the metabolic rate produced by metabolic depressors or 

 accelerators. 



Numerous carbon compounds present in the organism are metabolized 

 at a spectacular rate. Though, as shown by Bloch and Rittenberg^^^ 



Liver 



Infest. 

 Mucosa 



Control Urethane 

 Brain 



Muscles 



Fig. 15. — Effect of uiethane (20 mgm) on the incorporation of i*C 

 into tissue phosphatides. Mice killed 100 min, after injection of 



CH, • "CO,Na. 



a 100 gm rat by catabolizing fatty acids produces 1 gm of acetate and 

 other tissue constituents per day, the body acetate level of the organism 

 remains very low, owing to the rapid utilization of the acetate produced. 

 This compound is a precursor of acetoacetic acid, cholesterol, fatty 

 acids, glycogen, the dicarboxylic amino-acids, proto-porphyrin, uric 

 acid, and the acetyl group formed in many acetylation reactions, with 

 some of which the ^^C atoms of acetate are incorporated at a remarkable 

 rate. If we inject labelled acetate into the mouse, the injected radio- 

 active acetate is diluted by non-radioactive endogenous acetate. This 

 dilution rapidly increases owing to the fact that, while the radioactive 

 acetate injected only at the start of the experiment is rapidly metabolized, 

 the endogenous non-radioactive acetate, though rapidly metabolized 

 as well, is newly formed throughout the experiment. The decrease in the 



^54)/. Biol. Chem. 159, 45 (1945). 



63 Hevesv 



