AFRIKANDER CATTLE CONGENITAL GOITER - CHARACTERISTICS OF THE DEFECTIVE 19S THYROGLOBULIN SYNTHESIS

The characteristics of the defective thyroglobulin biosynthesis in genetically determined Afrikander cattle goiter were compared with normal thyroglobulin biosynthesis. In vivo biosynthesis was investigated by incubating goiter and normal thyroid slices with radioactive amino acids. The proteins in the incubation medium and in the soluble (100, 000 × g supernatant) and solubilized (digitonin extract of the 100, 000 × g precipitate) fractions were analyzed by sucrose gradient centrifugation and immunoprecipitation. While 19S thyroglobulin was clearly present and labeled in the fractions from normal thyroid slices, none of the goiter fractions contained a 280 nm-absorbing 19S peak. The radioactivity of all of the goiter fractions, however, overlapped the 19S zone and 20–40% of the counts in the 12 to 19Ssedimenting regions were precipitable by an antithyroglobulin double antibody system. By employing standardized methodology (Biochem Biophys Res Commun 78: 230, 1977), 33S mRNA was isolated from normal thyroids and goiter material. Both of the messengers were translated in vitro by a reticulocyte lysate cell-free system into 12–19S thyroglobulin-like material, which was judged to be very similar to authentic thyroglobulin by sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunoprecipitation. Differences in the primary structure of the normal and goiter translation products, as revealed by the tryptic peptide composition, might exist, however, and therefore need to be investigated in greater detail. Sucrose gradient analyses of the free ribosomes and membrane-bound polysomes extracted from normal thyroids and goiters showed that the goiter contained about twice as many free ribosomes as normal tissue, while its membrane-bound polysomes were smaller than normal in size. This difference is probably allied to the observations that in vitro translation of the goiter mRNA is performed with roughly the same degree of fidelity as the normal mRNA but, in vivo, a greatly reduced level of thyroglobulin-like protein is found. Therefore, it is suspected that the genetic defect of the goiter lies in the biosynthesis machinery responsible for the transfer of translation products across the membranes of the rough endoplasmic reticulum. © 1979 by The Endocrine Society.