Functions of Double Subunits of a Type, Structure of Iron Core, and Kinetics of Iron Release from Membrane Ferritin of Human Placenta

Abstract

Abstract Membrane ferritin of human placenta (HPMF) with electrophoresis purity was prepared in batch from human placental tissues. Several analysis methods were employed to study on the structure and functions of HPMF. The results of SDS-PAGE indicated that the protein shell of HPMF comprised different subunits, which belong to a similar type. The subunits were classified into two groups with the molecular weights of 15 and 20 kDa, which were named MF 15 and MF 20 subunits, respectively. In addition, relative amount of MF 15 subunit was about two folds more than that of M 20 subunit in SDS-PAGE gel. Both MF 15 and MF 20 subunits were identified by peptide mass fingerprinting (PMF) and showed high homology with L subunit of human ferritin. Accordingly, it was believed that HPMF was a novel ferritin of two subunits with different molecular weights belonging to a similar type. Using a matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS), the subunit stability of HPMF was investigated, and five mass peak's corresponding ratio of mass to charge ( m / z ) such as 5071.25, 9962.51, 15131.55 (MF 15 ), 19936.40 (MF 20 ), and 20147.61 were detected, which indicated HPMF had five subunit formula of [MF 15 ] 3+ , [MF 20 ] 2+ , [MF 15 ] + , [MF 20 ] + and [MF 20 + Fe] + , respectively. Using inductively coupled plasma-mass spectrometry (ICP-MS) to analyze the elemental composition of HPMF, only 85 Fe 3+ and 15 inorganic phosphate (P i ) ions in the iron core of per molecule of HPMF were determined, which were significantly lower than that of most mammal ferritins, but higher than that of bacterial ferritin (BF). The kinetic studies showed that iron release from the complete HPMF followed the regulation of first-order reaction, and the time taken for the whole process of iron release from HPMF was about 750 min, which was slower than that from both horse spleen ferritin (HSF) and pig pancreatic ferritin (PPF), which were about 60 min. Accordingly, based on the difference of structure and function among HPMF, mammalian ferritins, plant ferritins, and BF, we proposed that M 15 and M 20 subunits play different roles in iron transporting from maternal blood to fetal blood.