The synthesis and secretion of rat transferrin.

Transferrin, isolated from plasma or serum, had a molecular weight of 76,500 and its NHz-terminal amino acid sequence was Val-Pro-Asp-Lys-Thr-Val-Lys-Trp(Cys)-Ala-Val-Ser-Gluu-His-Glu-Asn-Thr-Lys-(Cys)-IleSer-Phe-Arg-Asp-His-Met-Lys-Thr-. About one-third of total serum transferrin contained 3 mol of iV-acetylneuraminic acid and about two-thirds of total contained 2 mol of N-acetylneuraminic acid/m01 of transferrin. The former had an isoelectric point of 5.65, the latter one of 5.85. About 1% of total transfer& in serum had an isoelectric point of 5.35 and a trace amount had an isoelectric point of 6.1. The half-lives in serum were similar for transfer& containing 2 or 3 Wacetylneuraminic acid residues. Characteristic differences in the labeling kinetics of transferrin, albumin, and al-acid glycoprotein with radioactive L-leucine or D-ghCOSan’Iine suggested that these proteins were processed independently in the liver cell. The secretion of transferrin was inhibited by proteinase inhibitors but not by inhibitors of glycosylation. A transferrin-like protein was isolated from liver. It possessed the same NH&erminal amino acid sequence as transfer& from serum but did not appear to contain N-acetylneuraminic acid. After injection of radioactive amino acids it was labeled earlier than the sialylated forms of transferrin. A cell-free protein-synthesizing system from wheat germ, programmed with RNA isolated from liver, synthesized a protein precipitable with anti-transferrin antiserum. Compared to serum transferrin, it possessed an NHz-terminal extension of 20 amino acids beginning with Met-Lys-. . . .