Structure, assembly, conformation, and immunological properties of the two subunit classes of ferritin.
The two subunit types of human liver ferritin were purified to homogeneity. Both subunits reassembled in a well-defined manner and formed spherical particles that resembled natural apoferritin in electron micrographs. Affinity chromatography methods were employed to obtain preparations of antibodies that interacted exclusively either with the H or with the L polypeptides, demonstrating that distinct immunological properties may be ascribed to each subunit of ferritin. The amino acid compositions of the subunits were similar, but the larger H subunit had fewer leucine, phenylalanine, and arginine residues. It is therefore improbable that H subunits undergo proteolytic processing and are precursors for L subunits. Circular dichroism data indicated that homopolymers assembled from L-type subunits had substantially more ordered secondary structures and greater alpha-helical contents than their H counterparts. Small differences in the environment of tryptophan residues were evident from fluorescence spectra of each homopolymer. In isoelectric focusing experiments reassembled H or L homopolymers migrated as families of proteins within discrete pI ranges which are probably representative of subpopulations of each subunit type. The H homopolymer focused at lower pI's than the L component. These data substantiate the contention that both subunits are authentic polypeptide moieties of ferritin with some common structural features, but the results also underscore prominent dissimilarities in their properties.
D001052:Apoferritins; D055598:Chemical Phenomena; D002621:Chemistry; D002942:Circular Dichroism; D005293:Ferritins / Q000145:classification / Q000276:immunology; D006801:Humans; D008099:Liver / Q000032:analysis; D008854:Microscopy, Electron; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
D005293:Ferritins; D001052:Apoferritins