72 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



2. Electron transfer: 



Fe++ + Fe*^+ ^ Fe*++ + Fe^+ (2-2) 



This type of exchange is known to occur by the transfer of electrons from 

 one substance to another. 



3. Atom transfer: 



ethyl 

 alcohol 



I*- + CH3I > I- + CH3I* (2-3) 



This reaction involves the Walden inversion mechanism, which has been 

 widely studied. 



4. SoHd-fluid reaction: 



Bone crystal-Ca + Ca*++ -> Bone crystal-Ca* + Ca++ (2-4) 



The implications of the exchange of ions between plasma and bone are 

 discussed in detail later. 



5. Solid-gas reaction: 



H2O 



vapor 



BaCOs + C*02 ^ BaC*03 + CO2 (2-5) 



Carbon 14 from experimental material is often converted to BaC'^Os 

 for radioassay. The loss of activity from such BaC^^Os samples by 

 exchange with the CO2 of the air may be serious (42). Similar exchange 

 may interfere with various degradation procedures carried out to assign 

 the C'^ label to particular locations in the organic molecule (43). 



6. Reversal of a degradation reaction: 



inorganic 

 phosphate* 



Organic phosphate ^;:^^ organic residue -\- inorganic phosphate (2-6) 



Even though a substance may be undergoing degradation, there is the 

 possibility that the reverse reaction may occur to a slight degree. This 

 might result in the appearance of the label in the substance under study 

 but could not be interpreted as metabolic incorporation. 



Although these types of reactions have provided much valuable infor- 

 mation on reaction rates, bond strengths, and molecular rearrangements 

 and structure, the interest here is in connection with their disadvantages 

 in tracer studies. In problems of intermediary metabohsm, radiotracers 

 are used to provide information on the processes involved in the biological 

 synthesis of important metabolic compounds. If the radioactive element 

 becomes incorporated in the metabolic product merely by exchange, 

 which requires no energy production, then these experiments are of no 

 value in that there is no evidence for synthesis of the product. 



It is usually possible, however, to carry out studies that can show 



