Structural analysis of the asparagine-linked oligosaccharides of rat haptoglobin metabolically labeled in a hepatocyte culture system

Abstract

We analyzed the asparagine-linked oligosaccharide chains of rat haptoglobin which were synthesized and secreted by hepatocytes in primary culture. When the cells were incubated with either [3H] Haptoglobin is one of major plasma glycoproteins which are synthesized and secreted by hepatocytes [I]. Its function is to form a strong and stable complex with hemoglobin which has been released from erythrocytes and foster the recycling of heme iron [I, 21. Haptoglobin is a tetrameric protein composed of two types of polypeptide chains, a and p, which are covalently associated by disulfide bonds (a&). Recently it has been established that the protein is initially synthesized as a single polypeptide chain precursor, which is proteolytically processed to form the dissimilar c1 and # subunits of the native protein [3 -61. The post-translational proteolytic cleavage of the precursor is found to take place at an early stage of intracellular transport, possibly within the endoplasmic reticulum [4 -61. The / 3 subunit with M , 33 000 found just after proteolytic conversion is further modified by glycosylation to the mature form with M , 36000 before secretion [5].Previous observations indicate that haptoglobin has the carbohydrate moiety consisting of mannose, fucose, galactose, N-acetylglucosamine and sialic acid (N-acetylneuraminic acid), amounting to 10-20% by weight; 15-19% in human [7,8], about 20% in goat [9] and 14.1% in rat [lo]. The carbohydrate moiety is usually contained in the fl subunit [I, 81, although it is also detected in the c1 subunit in Correspondence to