Structure and function of ovotransferrin. I. Production of iron-binding fragments from iron-ovotransferrin by the action of immobilized subtilisin. Purification and characterization of the fragments.

Immobilized subtilisin Novo was used for the cleavage of iron-saturated ovotransferrin (Fe2OT) into separate NH2- and CO2H-terminal iron-binding fragments, designated as FeNF and FeCF, respectively. The Mr of each fragment is 39,000. The purified fragments show major differences in the content of histidine, alanine, and methionine. Both apo-NH2- and apo-CO2H-terminal fragments are able to bind one ferric ion per molecule. FeCF is more resistant than FeNF to dissociation at acid pH and to subtilisin action. FeNF and FeCF are immunochemically distinct. However, equal mixtures of the two show immunochemical reaction indistinguishable from intact Fe2OT. The iron-binding sites of FeNF and FeCF are very similar to each other on the basis of visible absorption and CD spectra. The major difference in the backbone conformations between FeNF and FeCF is in the alpha-helical content of FeCF which is twice that of FeNF. Individually, fragments show quantitative differences in the electron paramagnetic resonance spectra; however, equal mixture of the two fragments produce EPR spectra very similar to that of the intact Fe2OT. These studies indicate that subtilitic cleavage of Fe2OT does not produce significant change in the iron-binding capacity or the conformation of the separated iron-binding domains.