Properties of ferritin from the earthworm Octolasium complanatum

The isolation and characterization of a protein with a high iron content from a common Italian earthworm (Octolasium complanatum) is described. This protein represents the first example of a purified ferritin from annelids. O. complanatum ferritin migrates on polyacrylamide gel electrophoresis with a mobility similar to that of horse spleen ferritin, and has hydrodynamic properties, i.e., elution volume and sedimentation velocity, which likewise correspond to those of horse ferritins. It can be resolved in a family of isoferritins with isoelectric points ranging from 6 to 7; accordingly, SDS gel electrophoresis reveals the presence of two subunit types of M_r 19700 and 20500, which are present in roughly equal amounts. O. complanatum ferritin crystallizes from poly(ethylene glycol) and is immunologically unreactive with antisera against human or horse ferritin. Apoferritin can be prepared by means of reducing agents, just like mammalian apoferritins. It catalyses iron oxidation and incorporation inside the protein shell under conditions that are known to lead to ferritin reconstitution. The spectroscopic properties of O. complanatum apoferritin indicate that the aromatic residues are immobilized in a hydrophobic environment, as in the case of mammalian apoferritins. Moreover, the presence of dichroic bands in the region 320-400 nm suggests that an extrinsic chromophore copurifies with earthworm ferritin.