TRACER EXPERIMENTS WITH I131 LABELED HUMAN SERUM ALBUMIN: DISTRIBUTION AND DEGRADATION STUDIES

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Abstract
I131-labeled human serum albumin was administered intraven. to 46 subjects in 53 expts. Plasma, urine, fecal and ascitic fluid concns. of radioactivity were followed for periods up to 6 weeks. Various body sites were assayed by in vivo technics. The labeled albumin reached distribution equilibrium in 4-7 days except in subjects with massive ascites, in whom this period was prolonged. The apparent space of distribution in non-ascitic subjects averaged about 2.5 times the plasma volume. Two apparent rates of transfer between plasma and extravascular spaces, with half times of approx. 3 and 24 hours, were observed. Radioactivity released by degradation of the protein-iodine bond was almost completely accounted for by renal excretion in the presence of thyroid blockage. Where observation periods were adequate, independent detns. of degradation rates from plasma and urine data agreed well. Most lots of I131-labeled albumin utilized contained variable quantities of rapldly degraded components. The major components in the best lots were degraded at a rate of about 4% per day (T1/2 = 17 days), or less, which agrees well with values obtained by biosynthetic methods but not with those previously reported by this method. It is concluded that the validity of the I131-labeled album in as a tracer for the study of endogenous albumin "turnover" rates has not been unequivocally demonstrated. Reasons for the disagreement with results of previous studies along the same lines are discussed.