The nature and rates of excretion of radioactive breakdown products of I131-albumin in the rabbit.

When I(131)-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 I(131)-iodide and the remainder are organic I(131)-iodine compounds. When I(131)-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 I(131)-iodine compounds liberated during the metabolism of I(131)-albumin can be closely matched by a mixture of the organic I(131)-iodine compounds liberated during the metabolism of I(131)-monoiodotyrosine, I(131)-diiodotyrosine, and the amino acids released by digestion from I(131)-albumin. These organic I(131)-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 I(131)-iodide, and, to a satisfactory approximation, the same equations describing the excretion of I(131)-iodide with the same constants may be used for describing the excretion of the organic I(131)-iodine. These results permit improved estimates of the distribution and catabolism of I(131)-albumin.