Transcortin biosynthesis and intracellular distribution of rat liver polyribosomes synthesizing transcortin.

Transcortin biosynthesis in rat has been examined using liver slices technique. The incorporation of 14C-labeled amino acids into the anti-transcortin-precipitable material of liver slices has been measured and compared with that of serum transcortin. It was shown that liver synthesized transcortin with an apparent mol. wt of 66 kDa on SDS-electrophoresis which co-migrated with authentic rat serum transcortin. In order to determine an intracellular distribution of transcortin synthesizing polyribosomes, the binding character of [125J]anti-transcortin-IgG to free and membrane-bound rat liver polyribosomes has been studied. It was shown that after incubation of [125J]anti-transcortin-IgG with liver membrane-bound polyribosomes, the radioactivity was associated with the discrete polyribosome fraction in heavy polyribosome region. In similar experiments the radioactivity of [125J]anti-transcortin-IgG bound to the free polyribosomes was distributed throughout the polyribosome region. The results of our experiments obviously demonstrated that serum transcortin was synthesized exclusively on membrane-bound polyribosomes of rat liver; free polyribosomes were devoid of detectable antigenic material able to bind antibodies to transcortin.