Characteristics of structure, composition, mass spectra, and iron release from the ferritin of shark liver ( Sphyrna zygaena)

Abstract

The ferritin consists of a protein shell constructed of 24 subunits and an iron core. The liver ferritin of Sphyrna zygaena (SZLF) purified by column chromatography is a protein composed of eight ferritins containing varying iron numbers ranging from 400±20 Fe 3+/SZLF to 1890±20 Fe 3+/SZLF within the protein shell. Nature SZLF (SZLF N) consisting of holoSZLF and SZLF with unsaturated iron (SZLF UI) to have been purified with polyacrylamide gel electrophoresis (PAGE) exhibited five ferritin bands with different pI values ranging from 4.0 to 7.0 in the gel slab of isoelectric focusing (IEF). HoloSZLF purified by PAGE (SZLF E) not only had 1890±20 Fe 3+/SZLF E but also showed an identical size of iron core observed by transmission electron microscopy (TEM). Molecular weight of ∼21 kDa for SZLF E subunit was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Four peaks of molecular ions at mass/charge ( m/ z) ratios of 10611.07, 21066.52, 41993.16, and 63555.64 that come from the SZLF E were determined by matrix-assisted laser desorption ionization/time-of-flight mass spectrometry (MALDI-TOF MS), which were identified as molecular ions of the ferritin subunit (M +) and its polymers, namely, [M] 2+, [M] +, [2M] +, and [3M] +, respectively. Both SZLF E and a crude extract from shark liver of S. zygaena showed similar kinetic characteristics of complete iron release with biphasic behavior. In addition, a combined technique of visible spectrometry and column chromatography was used for studying ratio of phosphate to Fe 3+ within the SZLF E core. Interestingly, this ratio maintained invariable even after the iron release, which differed from that of other mammal ferritins.