Abstract
A vitamin B 12-protein complex was isolated from human amniotic fluid by (NH 4) 2SO 4 fractionation, ion-exchange chromatography, gel filtration isoelectric at pH values of 3.3, 3.7 and 3.9. The component isoelectric at pH 3.7 was homogeneous in analytical ultracentrifugation, disc electrophoresis, and immunodiffusion. The molecular weight determined by sedimentation equilibrium ultracentrifugation was 59 300. As the vitamin B 12-binding proteins in amniotic fluid and granulocytes had the same elution volume in gel filtration, and gave a reaction of complete identity in immunodiffusion, the isolated protein was classified as an R protein. The erroneously high molecular weight values of R proteins obtained by gel filtration, approximately 120 000, may be attributed to their high carbohydrate content, 33%.
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