Expression and study of recombinant rat liver ferritin heteropolymers

The cDNA sequences encoding the H and L chains of rat liver ferritin were simultaneously cloned into the baculovirus transfer vector pAcUW51, under the control of polyhedrin and plO promoters, respectively. Both ferritin chains were expressed and assembled into heteropolymers following infection of insect cells with recombinant virus. The percentage of H and L chains making up the heteropolymers, determined by gel scanning following SDS-PAGE, were equiv aient to ferritins containing 1, 4, 6, 8, 12, and 18 H chains per 24 subunits. The maximal extent of iron loading into these heteropolymers was similar (approximately 2100 atoms of iron per ferritin) using one mole of rat ceruloplasmin per mole of H chain in the ferritins. The same amount of phosphate was also incor porated after iron loading (approximately 410 atoms of phosphate per ferritin). Gradient-density centrifugation of native rat liver ferritin separated ferritin with different iron contents, however, the ratio of H to L chain ferritin in these fractions was similar. Ferritins isolated from rat livers and rat spleens, with 10 and 6 H chains per 24 subunits, respectively, were found to contain 1400 and 1850 atoms of iron per ferritin and 520 and 440 atoms of phosphate per ferritin, respectively. This agreed with the equation we reported earlier (de Suva et al. (1993) Arch. Biochem. Biophys. 303, 451-455), suggesting that the space in the ferritin iron core were similar. Therefore, the amount of iron loading and the space for ferritin iron core information were not related to different ratios of H and L chains in ferritin. (Supported by NIH grant ES05056.).