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BRIAN J. MORRIS, ELVIN S. P. CHENG, Clearance and Metabolism of 125I-Labeled Tonin in the Rat, Endocrinology, Volume 111, Issue 5, 1 November 1982, Pages 1462–1468, https://doi.org/10.1210/endo-111-5-1462
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125I-Labeled rat tonin was injected iv into male Wistar rats weighing 303 ± 17 g (mean ± SE), and the disappearance from the circulation, plasma interaction, tissue uptake, and metabolism of radioactivity was studied. After injection, 125I-labeled tonin was immediately bound by α1-macroglobulin, the major circulating protease inhibitor of rats, and the complex was cleared in a biexponential manner: t½ of the fast component = 1.8 ± 0.1 min; tfrac12 of the slow component = 84 ± 10 min; MCR = 2.1 ± 0.2 ml/min (mean ± SE, n = 8). The complex was taken up intact by tissues, the major ones being the liver, spleen, adrenal, thyroid, and kidney. Analysis of tissue extracts by gel filtration on Sephadex G-100 showed that the kidney was the major site of metabolism, and radioactivity similar in size to 125I appeared rapidly in urine. Therefore tonin circulates wholly as a complex with α1-macroglobulin (the rat equivalent of human a2-macroglobulin), which once formed is quickly eliminated from the bloodstream by tissue uptake and metabolized.