PART I -STEROIDS 



METABOLISM OF i^C-LABELLED STEROIDS* 



C. P. LEBLOND 



The basic postulate of tracer work is that the changes 

 exhibited by the radioactive material also take place normally 

 under physiological conditions. This is true only if the 

 amount of radioactivity is kept low enough to avoid radio- 

 chemical effects, and if the amount of the labelled substance 

 is such that it can mix with the same substance in body 

 fluids and tissues without significantly altering the physio- 

 logical level of the material under investigation. In the case 

 of ^^C labelled steroids, the first requirement is easily satisfied 

 but not the second. "f It is indeed quite difficult to trace 

 physiological amounts of ^^C-labelled substances which exhibit 

 their biological activity in minute doses, as is the case with 

 oestrogens. The problem, on the other hand, is somewhat 

 easier in the case of substances active in larger doses, for 

 example progesterone, with which we may hope to obtain 

 in the future truly physiological results. At any rate, a 

 greater amount of radioactivity per mg. of substance, i.e. a 

 higher specific activity, may be expected from (1) the use of 



♦Report from the Depts. of Biochemistry (R. D. H. Heard, F. Peron, J. 

 Saffron, L. Thompson and C. Yates) and of Anatomy (R. C. Greuheh and 

 C. P. Leblond), McGill University, Montreal. 



T Substances labelled with radiocarbon must of necessity contain a laroje 

 amount of carrier since the half life of ^^C is about 5,500 years, and therefore, 

 one millicurie must contain at least enough atoms to have 3-6x10' atoms 

 exploding every second over thousands of years. Thus, 1 mc. of radiocarbon 

 as barium carbonate must weigh at least 3 mg. In practice, the available 

 radiocarbon weighs several times that amount, due to contamination with 

 non-radioactive carbon in the course of preparation. As a result, the dose of 

 a ^^C-labelled substance which is administered for tracing purposes will often 

 be greater than the amoimt of the natural substance present in body fluids. 



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