Lipoprotein Synthesis in the Cockerel Liver: Effects of Estrogen on Hepatic Polysomal Messenger Ribonucleic Acid Activities for the Major Apoproteins in Very Low and High Density Lipoproteins and for Albumin and Evidence for Precursors to These Secretory Proteins*

The effects of estrogen on lipoprotein biosynthesis have been studied in the cockerel. After administration of 17β-estradiol (5 mg), plasma apoVLDL-II, a major apoprotein in very low density lipoproteins, increases 18-fold in concentration over 48 h. Plasma concentrations of albumin and apoA-I, the major apoprotein in high density lipoproteins, were unchanged. When hepatic polysomes were isolated from the cockerels, estrogen was found to result in aggregation of polysomes within 6 h. Translation of polysomal RNAs in a wheat germ system in vitro showed that estrogen selectively stimulated the accumulation of apoVLDL-II mRNA in the polysomes. Analysis of the products of translation by slab gel electrophoresis demonstrated that apoVLDL-II, apoA-I, and albumin were all synthesized as larger putative precursors. N-Terminal sequencing of the apoVLDL-II synthesized in vitro using various labeled amino acids revealed a 23 amino acid residue extension consisting of the following sequence: Met-X-Tyr-X-Ala-Leu-X-X-Ala-X-X-Leu-Leu-Leu- X-X-X-X-X-X-X-X-X/Lys, where Lys was the N-terminal residue of plasma apoVLDL-II. Comparison of the partial sequence of the prepiece with other known sequences of prepeptides showed no obvious similarity, except for the abundance of hydrophobic residues. (Endocrinology106: 275, 1980)