Metabolism of ^^C-Labelled Steroids 5 



radiocarbon uncontaminated with non-radioactive carrier, 

 and (2) the labelHng of as many carbon atoms as possible. 



Two types of methods for tracing radioactive material in the 

 body exist. The first is by the Geiger Counter tube, by which 

 quantitative estimates are obtained with extracts of tissues 

 and excreta. The other technique consists of the recognition 

 of radioactivity by means of its effect on photographic 

 emulsion. In this case, the detector is the photographic 

 grain, the size of which makes it suitable for the microscopic 

 localization of radioactivity. The autoradiograph technique 

 consists of placing emulsion in intimate contact with a thin 

 section of tissues of animals sacrificed at various intervals 

 after administration of radioactive material. Recently, a 

 similar method has been applied to the localization of radio- 

 active spots in paper chromatograms. This method, which 

 makes it possible to identify substances with a precision com- 

 parable to that of melting point determinations, can be used 

 to locate infinitesimal amounts of a substance provided it 

 contains radioactivity and a sufficiently long exposure is 

 used. However, the use of both the Geiger counter and radio- 

 autographic techniques is complicated by the low specific 

 activity of the carbon present in radioactive steroids. Thus, 

 in using the Geiger counter, the easiest method with the soft 

 jS-rays of ^*C is to employ preparations of infinite thickness, 

 for which corrections for absorption are unnecessary. But 

 this method has a low sensitivity, and to detect the small 

 amounts present in tissues, thin samples may have to be used, 

 and complex corrections for absorption may have to be 

 made. In autoradiography the main difficulty is that the 

 preparations require very long exposures, a year in the case 

 of the slide illustrated in Figs. 1 and 2. 



A number of radioactive steroids have been synthesized: 

 progesterone labelled with ^*C in positions 21 or 3; cestrone 

 labelled in position 16; and deoxycorticosterone acetate 

 (DCA), labelled in positions 21 and 3. Thus, the chemical 

 part of the research is well advanced, more so than the 

 biological. There have been a few reports on the use of 



