30 : 7/ Tracer Techniques 



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is common to all living organisms, being found in many building blocks 

 including amino acids, purines, pyrimidines, porphyrins, and flavins. 

 Amino acids are the units polymerized to form proteins; thus, all pro- 

 teins contain nitrogen. Likewise, the nucleic acids, DNA and RNA, 

 contain purines and pyrimidines and hence, nitrogen. 



Naturally occurring nitrogen consists of the isotopes N 14 (99.64 per 



20 40 60 80 100 140 



Time in Days 



180 



220 



Figure 5. A tracer experiment. N 15 excess in heme of human 

 erythrocytes after feeding N 15 labelled glycine for 3 days. 

 After D. Shemin and D. Rittenberg, '"Life Span of the 

 Human Red Blood Cell," J. Biol. Chemistry 166: 627 (1946). 



cent) and N 15 (0.36 per cent). Because the natural abundance of the 

 N 15 is so much lower, it forms an ideal isotope for stable-isotope studies. 

 The longest-lived radioactive isotope, N 13 , has a half-life of only 10 

 minutes. Accordingly, it can be used only in a limited number of 

 experiments. The isotope N 15 is the one used for most biological 

 tracer studies involving nitrogen. 



One example discussed of the use of C 14 was the incorporation of the 

 amino acid glycine into protoporphyrin and hence into heme and 

 hemoglobin. Similar studies using N 15 -labeled glycine gave similar 

 results. The N 15 -labeled hemoglobin has then been used to study the 

 average lifetime of red blood cells. 



For these studies, humans were fed for 3 days on N 15 -labeled glycine. 

 As shown in Figure 5, the N 15 -excess built up rapidly in the heme in the 

 red blood cells. This is interpreted to represent the rate of "birth" 

 of new cells. For more than 60 days, few cells are destroyed. At 

 sometime around 80 days, the amount of excess N 15 decreases. Using 

 the curve shown in Figure 5, it is possible to compute the average life 

 span of human red blood cells as 127 days. These and similar experi- 

 ments with labeled Fe and with C 14 have confirmed this average life 

 span and also that the heme group is not re-used. 



