ISOTOPE TECHNIQUE 



Tims, <Y.n:'-(li(leiiteriosiiccinic acid in llir li\ino' cell inav lose all its 

 deuteiium by repeated dehydrogenations and hydrogenation (12). 

 The appearance of deuterium in the cellular fluids cannot be regarded 

 as proof that the succinic acid has been oxidized to carbon dioxide and 

 water. If, on feeding a deuterio compound to an animal, one isolates 

 another compound containing a higher concentration of deuterium 

 than exists in the body fluids, it»may be taken as positive proof of the 

 utilization of the first compound in the synthesis of the second. The 

 power and elegance of this piocedurc is illustrated by the finding that 

 the feeding of deuterioacetate to rats leads to the formation of deuterio- 

 cholesterol (3). There is no other method at present available which 

 could have disclosed such a complex synthetic reaction. It further 

 illustrates the biological stability of the C — D bond. 



While, at first sight, the fact that the C — D bond can be broken 

 in the course of chemical reactions (by dehydrogenations, enolizations, 

 etc.) seems to limit the field of applicability of deuterium, more careful 

 considerations suggest that this is, in actuality, ojie of the most useful 

 properties of deuterium. The same reactions which will remove 

 deuterium from an organic compound will also introduce it if the 

 medium in which the reaction occurs contains heavy water (D2O). 

 Injecting D2O into an animal makes possible the study of a host of 

 reactions and their rates, by determining the velocity of introduction 

 of deuterium into the organic compound. When the body fluids 

 of mice are enriched in D2O, the isotope concentration in the fatty 

 acids increases and finally reaches a value half that of the body fluids. 

 The isotope enters the fatty acid molecule in the course of the reactions 

 involved in the synthesis of the fatty acids. From such observations 

 (26) it was concluded that half the fatty acids of the mouse was de- 

 graded and resynthesized in about seven days. 



On the whole, it appears that deuterium is at present the most 

 valuable single isotope available to the biochemist. In the use of 

 heavy nitrogen as a label for nitrogenous compounds,' it must also be 

 remembered that reactions exist which may detach the labeled atom 

 and transfer it to another molecule. This is of course especially true 

 in the case of an amino acid. The use of carbon for labeling organic 

 compounds is not .subject to such uncertainty becau.se no exchange 

 reactions occur. Further, the conversion of one organic compound 

 to another is, by definition, the utilization of the whole or part of the 



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