568 FRED M. U B E R 



Much valuable work has been done recently on the carbon dioxide 

 metabolism that occurs almost universally in biological systems. 

 Perhaps because this kind of reaction could not be readily studied ex- 

 cept after the advent of carbon isotopes, it has enjoyed unusual popu- 

 larity the past few years. Since the results have been discussed 

 rather widely elsewhere, it seems sufficient here merely to refer to a 

 recent review (28) . 



Nitrogen metabolism within the animal body has been investi- 

 gated extensively by a number of workers, but especially in the long 

 series of researches by Schoenheimer, Rittenberg, and associates. 

 Reference must be made to their reviews {23,24) for an adequate ac- 

 count of this work, which covers not only a great many of the amino 

 acids but also other nitrogen compounds. 



3. Rates of Biological Processes 



The use of isotope labels has facilitated rate measurements for 

 numerous biological processes. A suggestive list of possibilities 

 could include the following : the rate of absorption of nutrient constit- 

 uents by various tissues, the excretion rate of waste products either 

 across permeable membranes or from an organism as a whole, the 

 rate of metabolic turnover for various elements and compounds, the 

 rate of deposition or accumulation of specific substances in definite 

 locations within a plant or animal, and the rate of circulation of body 

 fluids. 



Simple exchange reactions constitute a group of special interest 

 with respect to the use of isotopes since their rates cannot be measured 

 by any other method. Further, the presence of an exchange reaction 

 that involves the isotopic element used as a label will often disqualify 

 the use of a particular compound for a given investigation. This is 

 discussed in greater detail in a later section. 



Many biological studies are conducted in which the organisms are 

 maintained in an equilibrium or steady state condition. Isotopes 

 are particularly applicable in such cases since they permit interesting 

 reaction rates to be measured quantitatively even in the absence of 

 any demonstrable net change in chemical constitution of the system. 



4. Quantitative Analysis 



The quantitative analysis of a multicomponent solution by the 

 stable isotope dilution technique furnishes an excellent illustration of 



