Porcine serum cobalophilin and transcobalamin. Identification, isolation and properties including electrofocusing...

Pooled porcine serum was found to contain cobalophilin (also called transcobalamin I) and transcobalamin (also called transcobalamin II). The two proteins were harvested by batchwise absorption with vitamin B-12 covalently coupled to Sepharose, and then separated from each other either by gel filtration or using an immunoadsorbent. Both proteins were finally isolated as single proteins using a second vitamin B-12-Sepharose chromatography step. Cobalophilin and transcobalamin complexed with vitamin B-12 had molecular weights by gel filtration of 135 000 and 38 000 and by the formula of Svedberg 104 000 and 44 000, Stokes radii 4.97 nm and 2.65 nm, and sedimentation coefficients 5.39 S and 3.75 S, respectively. Electrofocusing resolved the cobalophilin complex into three main isoproteins isoelectric at pH 3.23, 3.42 and 3.69, and transcobalamin into only the main component isoelectric at a value as low as pH 3.47. Neither protein was capable of binding to the ileal intrinsic factor receptor.