ISOLATION AND CHARACTERIZATION OF RABBIT SERUM AND MILK TRANSFERRINS - EVIDENCE FOR DIFFERENCE IN SIALIC ACID CONTENT ONLY

Transferrin has been isolated from rabbit serum by diethylaminoethyl Sephadex chromatography and electrophoresis. A similar iron-binding protein has been isolated from rabbit milk whey by electrophoresis and gel filtration. The two proteins are readily crystallized from distilled water at pH 5.3. Measurements have been made of nitrogen and iron content, molecular weight, light absorption spectra in the visible and ultraviolet ranges, and amino acid composition. The proteins have also been compared by electrophoresis on cellulose acetate and starch gel, by double diffusion in agar against specific antisera, and by twodimensional peptide mapping of tryptic digests. The two proteins appear identical by all of these methods of analysis except electrophoresis, but this difference is eliminated by treatment with neuraminidase. The changes in mobility of the proteins after treatment with neuraminidase suggest that in serum most transferrin molecules contain two sialic acid residues while a few have only one residue; however in milk most molecules probably have one sialic acid residue, with a small proportion of the molecules having two. The molecular weight of the protein observed in the ultracentrifuge and on gel filtration calculates to be 70,000 daltons. The number of peptides observed on the maps, when considered in conjunction with the amino acid composition, suggests that this protein consists of two subunits. © 1968, American Chemical Society. All rights reserved.