. Biophysics: concepts and mechanisms. Biophysics. 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.
. Biophysics: concepts and mechanisms. Biophysics. 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 I131 and in
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