SUMMARY: pellicle, separated by sucrose density gradient centrifugation, had a high cobalamin (Cb1) binding activity. Of the Cb1 binding protein 65% was solubilized from pellicle by using 0.1% SDS/2 M-urea and the residual 35% by using 1% (w/v) SDS. Both of the Cb1 binding protein fractions showed a broad pH dependence for the binding of Cb1. No evidence for the involvement of SH-groups or metal ions in Cb1 binding was obtained. The values for cyanocobalamin of the proteins solubilized with 0.1% SDS/2 M-urea and with 1% SDS were 0.22 and 0.19 nM, respectively. The of a Cb1 binding polypeptide of each protein was estimated to be 38000 by SDS-PAGE. The Cb1 binding protein solubilized with 0.1% SDS/2 M-urea was purified to homogeneity. Inhibition experiments on Cb1 uptake in cells, using an antibody against the Cb1 binding protein solubilized with 0.1% SDS/2 M-urea, showed that the Cb1 binding proteins solubilized with 0.1% SDS/2 M-urea and 1% SDS take part in the slower secondary phase and in the initial rapid phase of Cb1 uptake in , respectively.


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