The Nature and Rates of Excretion of Radioactive Breakdown Products of I131-Albumin in the Rabbit

Abstract

When I131-albumin is given intravenously to rabbits, the radioactive breakdown products that are released into the plasma and urine can be extracted into acetone. Paper chromatography and paper electrophoresis show that about 80 per cent of these are I131-iodide and the remainder are organic I131-iodine compounds. When I131-iodide is given to rabbits taking iodide in their drinking water, the radioactivity is quantitatively excreted, without being accumulated in the tissues and without becoming attached to the plasma proteins. The rate of excretion can be defined by a first order rate process with a rate constant, a, ranging between 1 and 3day-1. The organic I131-iodine compounds liberated during the metabolism of I131-albumin can be closely matched by a mixture of the organic I131-iodine compounds liberated during the metabolism of I131-monoiodotyrosine, I131-diiodotyrosine, and the amino acids released by digestion from I131-albumin. These organic I131-iodine compounds are not accumulated in the body and their radioactivity does not become attached to the plasma proteins. Their radioactivity is excreted as fast or faster than that of I131-iodide, and, to a satisfactory approximation, the same equations describing the excretion of I131-iodide with the same constants may be used for describing the excretion of the organic I131-iodine. These results permit improved estimates of the distribution and catabolism of I131-albumin.