Comparative investigations on thyroglobulin biosynthesis in rabbit and guinea-pig thyroid in vitro.

The dynamics of biosynthesis of thyroglobulin (TG) subunits and their polymerization were studied comparatively in rabbit and guinea-pig thyroid in vitro. The incorporation of 14C-leucine into soluble, microsome-bound proteins (total and per mg) and into various fractions of soluble proteins was followed during incubation for 5 to 300 minutes. Some differences in TG synthesis between rabbits and guinea-pigs were observed, namely in the dynamics of 14C-leucine incorporation, the rate of subunit aggregation, the relationship between soluble and particulate proteins and the sedimentation characteristics of newly synthesized and pre-formed TG. The incorporation of 14C-leucine into soluble and microsome-bound proteins increased with time of incubation, but in rabbits the level of incorporation was much higher than in guinea-pigs. In rabbits, the ratio between radioactivity in soluble and particulate proteins decreased with time. In guinea-pig, this ratio increased with time of incubation and was much higher than in rabbits. Sucrose density gradient analysis of the newly formed soluble proteins showed that 12S fraction was synthesized in relatively large amounts, during early incubation of rabbit thyroid, although it was no found as a native protein in the thyroid extract of this species. After prolonged incubation the quantity of 12S protein decreased with a simultaneous increase in the amount of newly formed TG. The quick transformation of 12S subunit into TG is probably the reason why native 12S protein is absent in rabbit thyroid gland. However, in guinea-pigs the transformation into TG is much slower so that a considerable amount of radioactivity remained in the 12S fraction even after 300 min incubation. The results from rabbit experiments suggest that the 12S protein, probably represents a precursor of TG. In guinea-pig, the origin of 12S protein is twofold, as subunit of TG and as a product of its dissociation.