Changes in the ultraviolet and circular dichroism spectra of aquo-, hydroxy-, azido-, and cyanocobalamin when bound to human intrinsic factor or human transcobalamin I.

The ultraviolet and the circular dichroic spectra of aquo-, hydroxy-, azido-, and cyanocobalamin free or bound to purified human intrinsic factor or purified human transcobalamin I were recorded. Except for azidocobalamin-transcobalamin I, the molar absorption for the gamma1-band in the ultraviolet spectra increased with a factor between 1.2 and 1.4 when the cobalamins were bound to protein. The ultraviolet spectrum of azidocobalamin-transcobalamin I changed so that the gamma2-band became the most intense. Minor changes were observed in the other ultraviolet spectra. The most prominent change in the CD-spectra of protein bound cobalamins was the increase in negative elipticity above 400 nm. This increase was most intense for cyano- and azidocobalamin-transcobalamin I. The study emphasizes that the mechanism by which human intrinsic factor and human transcobalamin I bind cobalamins differ.