Some Properties of a Binding Protein Specific for Asialoglycoproteins and Its Distribution in Rat Liver Microsomes

Abstract

A binding protein specific for asialoglycoproteins was isolated from rat liver by affinity chromatography. The protein is composed of a single glycoprotein with an estimated MW of 52,000 in which sialic acid, galactose, mannose and glucosamine make up .apprx. 10% of the total molecule. The binding activity of the rough microsomes was 1/3 that of the smooth microsomes. Rough and smooth microsomes were digested with trypsin and neuraminidase and incubated with the antibody to the binding protein in the absence or presence of Triton X-100. Without Triton X-100, 25-30% and .apprx. 50% of the binding activity of the rough and smooth microsomes, respectively, were always protected from digestion by the enzymes or were freed from inhibition by the antibody. In the presence of Triton X-100, trypsin inactivated 100%, but neuraminidase inactivated .apprx. 80% and .apprx. 70% of the binding activity of the rough and smooth microsomes. The binding activity on the luminal side of the microsomal membrane may be due to the precursor form(s) of the bidning protein.