Lactoferrin and transferrin: A comparative study

Abstract

The optical and electron paramagnetic resonance (EPR) spectroscopic parameters which characterize the two metal-binding sites of lactoferrin are strikingly similar to those of transferrin. Ultraviolet difference spectra indicate the involvement of tyrosyl residues in iron binding, while EPR studies of the Cu(II)-lactoferrin-bicarbonate complex demonstrate the presence of at least one nitrogen ligand at each metal-binding site. However, the K 1 for Fe(III) binding as obtained by equilibrium dialysis and corrected for H + effects, is 300 times larger for human lactoferrin (measured near pH 6.4) than for human transferrin (measured near pH 6.7). Fe(III) binding to both proteins appears to involve equivalent and non-interacting sites. The disulfide structure of lactoferrin and transferrin also appear to be similar, as shown by gel-filtration studies. This, together with the nearly identical molecular weights of the proteins, their single-chain nature, metal-binding sites, corroborate the suggestion that the transferrins and lactoferrins constitute a single class of proteins evolving from a common ancestral protein.