Structural features of bovine fetuin revealed from analysis of the primary translation product: anomalous behavior on sodium dodecyl sulfate-polyacrylamide gel electrophoresis is due largely to peptide and not solely to carbohydrate.

Native bovine fetuin is a major alpha 1-glycoprotein in the serum and cerebrospinal fluid of fetal calves. We previously reported (Johnson, W.V., and Heath, E.C. (1986) Biochemistry 25, 5518-5525) the purification of the primary translation product for fetuin (prefetuin) from a rabbit reticulocyte cell-free translation system and showed that prefetuin contains an 18 amino acid signal peptide. Here we report that although the apparent sodium dodecyl sulfate (SDS) gel molecular weights of fetuin and prefetuin are 64,000 and 49,000, respectively, when analyzed by gel filtration chromatography under denaturing and reducing conditions, molecular weight values of 48,000 and 40,000 were found for native fetuin and the nonglycosylated prefetuin, respectively. These molecular weight values are in agreement with those expected on the basis of the sedimentation diffusion data of Spiro (Spiro, R.G. (1960) J. Biol. Chem. 235, 2860-2869) and indicate that the polypeptide moiety makes a major contribution toward the anomalous SDS gel electrophoretic mobility of fetuin. Therefore, the carbohydrate moiety is not solely responsible for this property. Edman degradation of [35S]methionine-labeled prefetuin indicated that the N-terminal residue is the only methionine present in prefetuin; native fetuin lacks methionine. Additionally, hydroxylamine cleavage of an Asn-Gly bond in prefetuin localized one of the N-linked carbohydrate side chains to the middle of the polypeptide chain of native fetuin.