1). RriTENBERG AND D. SHEMIN 



The essence of tiic isotope technique in biochemical research 

 consists in the preparation of a compound in which one or more of the 

 atomic comiDonents have an abnormal isotope concentration. In 

 t[iis manner, a substance is obtained which, by suitable isotope analysis, 

 can be detected and estimated quantitatively in the presence of either 

 its normal analogue or of other compounds, and which behaves like 

 its normal analogue in a biochemical system. 



Since the electronic configurations of the isotopes of an element 

 are almost identical, isotopic compounds have similar chemical propei'- 

 ties. There are, however, some physical properties which are de- 

 pendent on the molecular mass. Diffusion is one such property, since 

 the rate of this process is inversely proportional to the square root of 

 the mass. Nevertheless, these differences in physical properties are 

 not of importance in cellular metabolism. Molecules of glycine, 

 heavier by one unit of mass because of the introduction of one atom 

 of a heavy isotope of either hydrogen, nitrogen, or carbon, will differ 

 from the normal analogue in their rate of diffusion by about 0.7%. 

 There is direct evidence available that the cell does not distinguish 

 between isotopic isomers. Compounds containing one or more heavy 

 atoms of hydrogen, nitrogen, etc. normally exist in nature. For 

 example, 0.37% of all glycine molecules contain N^^ atoms instead of 

 N*^. Such compounds are therefore not foreign to the cell. If living 

 cells accorded special treatment to isotopic isomers, we should expect 

 the isotope concentrations to vary as we pass from tissue to tissue and 

 from the living into the inorganic world. However, the natural 

 abundances of the heavy isotopes of all elements studied are uniformly 

 the same in organic and in inorganic substances. It is striking that 

 this normal abundance of isotopes even applies to meteorites originat- 

 ing from interstellar space (21). 



The labeled compounds prepared must, of course, be of such a 

 nature that the isotope distribution will not be altered by mere ex- 

 change. Thus it is impossible to label ammonia in the water system 

 with either deuterium (Hj) or tritium (Hi), since the hydrogen atoms 

 of ammonia are readily replaced by those of the water. Several other 

 restrictions limit the scope of deuterium (33). When an organic 

 compound is labeled with deuterium, the carbon chain is not being 

 directly labeled, for deuterium may be lost from an organic compound 

 even though no reactions involving the carbon chain take place. 



266 



