Purification and characterization of rabbit haptocorrin.

Rabbit haptocorrin (R-binder) has been purified from serum by affinity chromatography on cobalamin-Sepharose and Blue Sepharose. It proved to be a protein with a relative molecular mass of 60 000 and an amino acid content very similar to that of other haptocorrins (Nexø, E. and Olesen, H. (1981) in B12 (Dolphin, D., ed.), Wiley Interscience, New York/London, in the press). The pH optimum (pH 6-9) for binding of cyanocobalamin and the affinity to dicyanocobinamide were like those of human and hog haptocorrins. In spectral studies, the extinction coefficient of cyanocobalamin at 363 nm (gamma 1-band) increased by about 16% on binding to rabbit haptocorrin. Binding of azidocobalamin gave spectra changes similar to those for binding to rabbit transcobalamin.