Metabolism of Iodinated Casein in the Rat

¹ From the Department of Physiology and Pharmacology, Michigan State University, East Lansing, Michigan

Radioactive (I¹³¹) iodinated casein was prepared under conditions established previously for maximum formation of thyroxine in such preparations. Iodide and loosely combined iodine were removed by dialysis against flowing tap water. Tissue distribution studies of radioactivity 100 minutes after intravenous injection of radioactive iodinated casein showed maximum concentration in the liver, followed in decreasing order by spleen, kidney, stomach, lung, small intestine and large intestine. In further experiments thyroparathyroidectomized rats were injected intravenously with iodinated casein solutions; blood and liver samples were obtained at varying intervals after injection, and the samples were fractionated into a) free and b) combined thyroxine-like compounds. Within 1 1/2 hours after injection both free and combined thyroxine-like compounds were found in both blood plasma and liver. Similar determinations at 24 and 72 hours showed the rapid disappearance of combined thyroxine-like compounds from plasma and their concentration in the liver. Simultaneously there was a pronounced increase in the ratio of free to total thyroxine-like compounds in plasma. These findings are believed to indicate that the liver is the principal site for the liberation of thyroxine from iodinated proteins when the latter are administered parenterally, and the free compounds are returned to the blood stream. The thyroidal action of iodinated proteins given by this route can then be explained as the action of the free thyroxine or related compounds that are so released.