40 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



Dornhorst (91), and Neuberger (92) has pointed out certain discrepancies 

 between results obtained from the aggkitination methods and those 

 obtained from the isotopic labeling methods. Burwell et al. (93) have 

 shown that lower values of red-cell life span in rabbits were found by 

 measurement of erythrocyte iron turnover than by direct measurement of 

 erythrocyte survival ; radioiron was employed in these studies. Necheles 

 et al. (94) have described the use of labeled sodium chromate (NaCr*^04) 

 for the estimation of survival of red blood cells. 



METABOLIC PATHWAYS 



It is not unexpected that the labyrinths of intermediary metabolism 

 would offer a most fertile field for radiotracer procedures. The biochem- 

 ist, organic chemist, and carbon 14 comprise a team that has contributed 

 greatly to elucidation of the various metabolic cycles. The general 

 approach involves the administration of isotopic compounds and the sub- 

 sequent isolation, purification, and measurement of possible conversion 

 products. Rigorous isolation procedures and the wide application of 

 degradation methods to allow assignment of the label to specific carbon 

 atoms in the molecule have allowed increasingly detailed interpretations 

 of the mechanisms involved. A broad base for advanced experimentation 

 is provided by the chemical synthesis of labeled compounds for use as 

 intermediates and by the biosynthesis of important naturally occurring 

 substances. The scope of investigations actively under way may be 

 gained from a consideration of some of the general fields in which carbon 

 isotopes are extensively used, which sounds like the roll call of the bio- 

 chemical sciences: animal and plant biochemistry, photosynthesis, micro- 

 biological metabolism, carbon dioxide fixation, fatty acid oxidation, gly- 

 colysis, fat metabolism, transmethylation, amino acids, and proteins. 



The literature dealing with metabolic pathways is already so formidable 

 and the rapid changing of status so overwhelming that no attempt can be 

 made to treat comprehensively even one narrow aspect. Reference to 

 the Annual Review type of article, in addition to consultation of standard 

 textual discussions, represents perhaps the best way of keeping abreast 

 of this field (9, 11, 12, 95 to 97). The following discussion on the pathway 

 of carbon in photosynthesis, based primarily on the work of Calvin and 

 coworkers (98, 99), illustrates the power of the method: 



Photosynthesis, the process by which green plants convert radiant 

 energy (light) into chemical energy (organic compounds), may be repre- 

 sented by the over-all equation 



light 



CO2 + H2O > [CHoO] + Oo (1-35) 



For an understanding of the mechanism it is necessary to know the path- 



