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Isolation of vitamin B12-binding proteins using affinity chromatography. 3. Purification and properties of human plasma transcobalamin II.

Abstract Transcobalamin II has been isolated from Cohn Fraction III derived from 1,400 liters of pooled human plasma, using affinity chromatography on vitamin B12-Sepharose and several conventional purification techniques. The final preparation was purified 2 million-fold relative to human plasma with a yield of 12.8% and was homogeneous based on polyacrylamide disc gel electrophoresis, sedimentation equilibrium ultracentrifugation, and chromatography on Sephadex G-150. Transcobalamin II binds 28.6 µg of vitamin B12 per mg of protein and contains one vitamin B12-binding site per 59,500 g of protein as determined by amino acid analysis. The molecular weight determined by sedimentation equilibrium ultracentrifugation was 53,900 and by gel filtration on Sephadex G-150 was 60,000. Sodium dodecyl sulfate polyacrylamide gel electrophoresis disclosed two peptides with molecular weight values of 38,000 and 25,000 which suggests that transcobalamin II contains 2 subunits. When vitamin B12 binds to transcobalamin II there is a shift in the peak of vitamin B12 absorption from 361 nm to 364 nm. Analysis of transcobalamin II for carbohydrate content using gas-liquid chromatography and amino sugar analysis by the amino acid analyzer suggest that this trace plasma protein is not a glycoprotein.