Whole-body measurement of radioactivity as a means of following in vivo the degradation of I131-labeled proteins in mice

Mice were injected with I131-labeled human or bovine serum albumin and determinations made of the rate of loss of radioactivity a) from the whole body ( in vivo counting), b) from the blood (serum samples), and c) from the total trichloroacetic acid precipitable mouse proteins (following homogenization). Measurements by each method made more than 8 hours post-injection gave the same half life (14.5 ± 0.5 hr.) for I131 bovine serum albumin (I131 BSA) when the level of circulating iodide was sufficient to prevent thyroidal accumulation (I131 HSA had a half time of 21 hr.). The variations observed between the methods in the first 24 hours can be explained in terms of times required for the several compartments to reach secular equilibrium, and therefore whole-body measurements in vivo can be safely used to measure the rate of degradation of this protein when allowance for these factors is made.