Partial characterization of intra- and extracellular forms of the glycoprotein hemopexin in liver cell culture and cell-free translation

Primary cultures of rat hepatocytes produce extracellular and intracellular species of hemopexin. We examined the presence of high-mannose oligosaccharides and neuraminic acid residues in these species by comparing their electrophoretic mobility on SDS-PAGE before and after digestion by endoglycosidase H and neuraminidase. The predominant intracellular form was not susceptible to digestion by neuraminidase but was sensitive to endoglycosidase treatment, which digested it to a species with a molecular weight comparable to that of the sole hemopexin species produced by tunicamycin-treated hepatocytes and that produced by in vitro translation. By contrast, both the minor intracellular and the extracellular species of hemopexin were neuraminidase-, yet not endoglycosidase H-sensitive, and may be identical. It can be concluded that the intracellular precursor contains high-mannose type oligosaccharides which are processed to complex type oligosaccharides shortly before secretion of hemopexin.