Desorption/Ionization of Subunit and Kinetics of Iron Release Revealed with Matrix-assisted Laser Desorption Ionization-Time of Flight Mass spectrometry and Electronic Spectrum in Liver Ferritin of Aplysia

Determination of iron content and dispersity of intact ferritin by superconducting tunnel junction cryodetection mass spectrometry

Ferritin is a common iron storage protein complex found in both eukaryotic and prokaryotic organisms. Although horse spleen holoferritin (HS-HoloFt) has been widely studied, this is the first report of mass spectrometry (MS) analysis of the intact form, likely because of its high molecular weight ∼850 kDa and broad iron-core mass distribution. The 24-subunit ferritin heteropolymer protein shell consists of light (L) and heavy (H) subunits and a ferrihydrite-like iron core. The H/L heterogeneity ratio of the horse spleen apoferritin (HS-ApoFt) shell was found to be ∼1:10 by liquid chromatography-electrospray ionization mass spectrometry. Superconducting tunneling junction (STJ) cryodetection matrix-assisted laser desorption ionization time-of-flight MS was utilized to determine the masses of intact HS-ApoFt, HS-HoloFt, and the HS-HoloFt dimer to be ∼505 kDa, ∼835 kDa, and ∼1.63 MDa, respectively. The structural integrity of HS-HoloFt and the proposed mineral adducts found for both purified L and H subunits suggest a robust biomacromolecular complex that is internally stabilized by the iron-based core. However, cross-linking experiments of HS-HoloFt with glutaraldehyde, unexpectedly, showed the complete release of the iron-based core in a one-step process revealing a cross-linked HS-ApoFt with a narrow fwhm peak width of 31.4 kTh compared to 295 kTh for HS-HoloFt. The MS analysis of HS-HoloFt revealed a semiquantitative description of the iron content and core dispersity of 3400 ± 1600 (2σ) iron atoms. Commercially prepared HS-ApoFt was estimated to still contain an average of 240 iron atoms. These iron abundance and dispersity results suggest the use of STJ cryodetection MS for the clinical analysis of iron deficient/overload diseases.

Cloning analysis of ferritin and the cisplatin-subunit for cancer cell apoptosis in Aplysia juliana hepatopancreas

Ferritin, an iron storage protein, plays a key role in iron metabolism in vivo. Here, we have cloned an inducible ferritin cDNA with 519 bp within the open reading frame fragment from the hepatopancreas of Aplysia juliana (AJ). The subunit sequence of the ferritin was predicted to be a polypeptide of 172 amino acids with a molecular mass of 19.8291kDa and an isoelectric point of 5.01. The cDNA sequence of hepatopancreas ferritin in AJ was constructed into a pET-32a system for expressing its relative protein efficiently in E. coli strain BL21, under isopropyl-β-d-thiogalactoside induction. The recombinant ferritin, which was further purified on a Ni-NTA resin column and digested with enterokinase, was detected as a single subunit of approximately 20 kDa mass using both SDS-PAGE and mass spectrometry. The secondary structure and phosphorylation sites of the deduced amino acids were predicted using both ExPASy proteomic tools and the NetPhos 2.0 server, and the subunit space structure of the recombinant AJ ferritin (rAjFer) was built using a molecular operating environment software system. The result of in-gel digestion and identification using MALDI-TOF MS/MS showed that the recombinant protein was AjFer. ICP-MS results indicated that the rAjFer subunit could directly bind to cisplatin[cis-Diaminedichloroplatinum(CDDP)], giving approximately 17.6 CDDP/ferritin subunits and forming a novel CDDP-subunit. This suggests that a nanometer CDDP core-ferritin was constructed, which could be developed as a new anti-cancer drug. The flow cytometry results indicated that CDDP-rAjFer could induce Hela cell apoptosis. Results of the real-time PCR and Western blotting showed that the expression of AjFer mRNA was up-regulated in AJ under Cd(2+) stress. The recombinant AjFer protein should prove to be useful for further study of the structure and function of ferritin in Aplysia.