Studies on the biosynthesis of rabbit haptoglobin

Rabbit haptoglobin is a tetrameric protein consisting of two nonglycosylated α and two glycosylated β chains, the latter being joined to the former and the former to each other by disulfide linkages. We describe here the results of studies that analyzed the biosynthetic pathway of rabbit haptoglobin by using cultured hepatocytes incubated with L-[³⁵S]cysteine. The initial form of haptoglobin detected in hepatocytes exhibited M(r) = 46,000, was glycosylated, and corresponded in migration to the initial species formed when the mRNA for rabbit haptoglobin was translated using the reticulocyte lysate system coupled with dog pancreatic microsomes. This one-chain intermediate was rapidly cleaved into a glycosylated form of the β chain and into the mature α chain, these chains being joined by disulfide linkages. Dimerization also occurred rapidly, forming a tetrameric precursor of haptoglobin. Several other intracellular glycosylated forms of the β chain were detected subsequently, representing intermediates formed during oligosaccharide processing prior to secretion of mature haptoglobin. Addition of tunicamycin (5 μg/ml) inhibited glycosylation of the initial form of haptoglobin detected, but subsequent proteolytic processing into α and β chains occurred. Our results show that the pathway of biosynthesis of rabbit haptoglobin closely resembles that reported for rat haptoglobin (Hanley, J.M., Haugen, T.H. and Heath, E.C. (1983) J. Biol. Chem. 258, 7858-7869).