648 ADOLF F. VOIGT 



read may supersede or make obsolete any specific suggestions made 

 here. One thing is clear, it is much better to get experience with 

 tracers by handling the safe ones at low levels first and after that to 

 progress to higher levels and more dangerous isotopes (3, p. 169; 

 69). 



I. BIOLOGICAL APPLICATIONS OF RADIOACTIVE 



TRACERS 



To discuss, even briefly, the vast number of applications of radio- 

 active tracers to biological problems that have appeared in print to 

 this date would require a chapter much longer than the present one. 

 It is not possible in all cases to draw a line between stable and radio- 

 active tracer research. Thus, in the case of carbon, early tracer 

 work with C^^ and continuing work with C^^ are intimately associated 

 with the increasing amount of work using C^^, now that it is widely 

 available. Research using tracers is the subject of a number of re- 

 views published recently to which readers are referred for additional 

 information and references to the original literature {70,71). 



In the case of the hydrogen isotopes, very little has been done with 

 tritium because it is unavailable and difficult to measure. The work 

 with deuterium is reviewed in Chapter XVI on stable isotopes. 



There is obviously more interest in tagging carbon atoms than 

 any others and the availability of the several carbon tracers has al- 

 ready led to increased knowledge in a number of biological problems. 

 The basic problems of photosynthesis and metabolism have duly re- 

 ceived a large share of this research (1, p. 148; ^, p. 182 ff., 3h,72-75). 

 References to some of the survey articles are given in the preceding 

 chapter since much of this research was done with C^^. Some idea 

 of the amount of work accomplished and that yet to be done can be 

 obtained from these reviews. Typical of other uses of radiocarbon 

 are the studies of individual biological reactions, as for example that 

 showing that phenylalanine is the precursor of epinephrine {76) . 



One of the most important radioactive tracers is P^^ by reason of 

 its half-life and other characteristics as well as its biological impor- 

 tance {1, p. 184; 2, pp. 209, 261; 71). Since many metabolic processes 

 involve phosphorus in the form of phosphate esters, P^^ has been 

 widely used in the study of metabolism of carbohydrates, phospho- 

 lipides, and nucleic acids. Other studies have dealt wdth other en- 

 zymic reactions, with the mechanism of insulin action, and with the 



