Membrane-bound thyroglobulin in an experimental thyroid tumor of rats

Abstract

Soluble and particulate iodoproteins have been studied in an experimental rat thyroid tumor (Wollman Line 1-1C 2) labeled by intraperitoneal injection of 125I-. The uptake of the tumor was very low (approx. 1% of the dose per g), and most of the radioactivity (75–95%) was particulate. The particulate iodoproteins were solubilized (10–78%) by vigorous rehomogenization and digitonin treatment; the soluble and solubilized proteins were analyzed by density gradient ultracentrifugation, microimmunodiffusion and double immunoprecipitation. Labeled 19-S thyroglobulin comprised 30–60% of the soluble iodoproteins and 10–90% of the total solubilized iodoproteins. In the iodoproteins solubilized by rehomogenization there was 10–23% 19-S at 60 min or less after injection, and most of the 125I sedimented at <6 S. This “light fraction” had a density similar to that of endoplasmic or plasma membranes and yielded 25–75% 19-S thyroglobulin when treated with digitonin. Subcellular fractionation showed that 62% of the total tumor 125I was in the “nuclear” fraction, 35% in the “microsomal” fraction and only 3% in the “mitochondrial” fraction, which should contain lysosomes. In vitro experiments failed to show non-specific adsorption of thyroglobulin as a source of membrane-bound thyroglobulin. The results suggest that in the tumor studied there is decreased synthesis of thyroglobulin and a possible defect in its release from cell membranes.