USES AS BIOLOGICAL TRACERS 



121 



acids are continually being degraded, and being replaced by syntheses. 

 That the rates of breakdown and resynthesis are the same is attested by the 

 fact that the concentrations are maintained constant during life. About 60 

 per cent of N ,5 -containing protein has been shown to appear as glycine in the 

 urine within 24 hr after a high-protein diet has been eaten; about 80 per cent 

 appears within 60 hr. Liver, plasma, and intestinal-wall proteins are re- 

 generated much faster than those of muscle and connective tissue. 



The nitrogen that goes into ringed structures such as the porphyrins, 

 which enter complexes with Fe +2 and Fe +3 to form the hemin of red-blood 

 cells, turns over quite slowly: it takes 10 days for the hemin to be synthesized 

 from isotopically tagged glycine, and then nearly 140 days before the deg- 

 radation process (cell replacement in this case) reduces the concentration of 

 tagged nitrogen to half the peak concentration (see Figure 5-10). 



indirect 



Time after oral administration 



Figure 5-10. Radioactivity in a Particular Vol- 

 ume of Tissue as a Function of Time After 

 Administration. Time and height of the maxi- 

 mum depend upon location of the volume, upon 

 what chemical compound is given, its normal 

 biochemistry, where it was introduced (direct 

 or indirect), and the half-life of the isotope. 



Other uses of radioactive tracers include the investigation of the effects of 

 drugs and hormones on the turnover rate in particular tissues or organs. A 

 subject of particular interest in recent years has been the role of insulin in 

 the control of diabetes. In a diabetic, sugars are transported across the 

 membrane and into the cell abnormally slowly, and they accumulate in the 

 plasma, useless for supplying energy, via oxidation, inside the cell. Insulin, 

 a medium-sized protein molecule whose structure has been well known since 

 it was first synthesized in 1956, has been tagged with I 131 and introduced 

 into the blood stream. Within minutes, more than a third accumulates in 

 the liver and the kidneys. However, a fraction adsorbs in a nonspecific man- 

 ner on all membranes accessible to blood plasma and intracellular fluids. 

 Cell walls are no exception; and the adsorption of insulin has been associ- 



