PRINCIPLES OF TRACER METHODOLOGY 



39 



ratively. Glycine labeled with the stable isotope iV'-' was used in the 

 original study, and the findings were later confirmed with C^Mabeled 

 glycine. The A^^Mabeled glycine was administered to a human adult, 

 and the N^^ concentration was followed as a function of time in the 

 various nitrogenous metabolic constituents. Except for the hemin iso- 

 lated from the red blood cells, all the constituents reached a peak value 

 at 24 hr after administration with a decline in N'*^ concentration there- 

 after. The hemin, however, rose rapidly for about 20 days, remained at 



Precursor 



/■) ti Time 



Fig. 1-8. Specific activity-time relations between product and precursor. [From 

 D. B. Zilversmit, C. Entenman, and M . C. FiMer, On the Calculation of " Turnover 

 Time" and "Turnover Rate" from Experiments Involving the Use of Labeling Agents, 

 ./. Gen. Physiol., 26: 325-331 (1943).] 



a plateau for the next 70 days, and then fell along an S-shaped curve. 

 These data were interpreted to mean: (a) The hemoglobin must have 

 become labeled during the formation of the red blood cell. (6) The 

 hemoglobin was not involved in a dynamic process or in the flux of syn- 

 thesis and degradation, (c) The hemoglobin remained in the red cell 

 until the cell disintegrated, (d) The heme was not reutilized. (e) The 

 red cells were not randomly destroyed, but their destruction was a func- 

 tion of age. It was also possible, from the curve, to estimate the average 

 life span of the red cells, which was found to be 127 days. Further studies 

 on the time course of specific activities demonstrated that glycine was the 

 precursor of the protoporphyrin of hemoglobin. The e(iuations for the 

 kinetics of this system may be found in the original papers. 



A critical discussion of red-cell-survival curves has been presented by 



