XVI. STABLE ISOTOPES AS TRACERS 595 



exceptional in that it does not react with deuterium in vivo. An ex- 

 tensive bibhography has recently been published {3 J a). 



2. Carbon 



Recent isotopic studies in the field of intermediary carbon metab- 

 olism have emphasized the fixation of carbon dioxide, a reaction all 

 but inaccessible to other methods of approach. The part played by 

 C>^ in the elucidation of the mechanism of carbon dioxide fixation has 

 been presented in an excellent review by Wood (38). Isotopic car- 

 bon investigations of metabolic mechanisms in general, including fats, 

 have been reviewed at length recently by several workers {32a; 

 27; see also 3). The transport of nutrients as well as the products 

 of photosynthesis has been investigated in plants with C*^ a guide 

 to these studies occurs in an article by Rabideau and Burr {26). 

 Only a beginning has been made in attacking the problem of photo- 

 synthetic reaction mechanisms and the chemical nature of the inter- 

 mediates {22a; 27, pp. 121-122). 



3. Nitrogen 



The important role of nitrogen in amino acid and protein metab- 

 olism has been actively investigated with N^^ ever since this isotope 

 was made available. Most of the work has been carefully reviewed 

 from time to time. The early studies are covered in articles by 

 Schoenheimer, both alone {23) and in collaboration with Rittenberg 

 {24). Later reviews have also appeared {29). Nitrogen fixation 

 by plants has also been reviewed recently {31). Satisfactory contact 

 with current progress in the field can be maintained only by consulting 

 the most recent annual review or the expanding literature in the jour- 

 nals. The fact that only a brief space is devoted here to directing 

 attention to N^^ studies on the translocation and intermediary metab- 

 olism of proteins, amino acids, and related compounds should not be 

 construed as an appraisal of their value. The results are regarded 

 generally as being of tremendous importance. 



4, Oxygen . 



Apart from numerous studies on oxygen exchange reactions of or- 

 ganic compounds {15), few biological applications of oxygen isotopes 

 have been made. Too rapid exchange is the principal reason. The 



