Two different H-type subunits from pea seed (Pisum sativum) ferritin that are responsible for fast Fe(II) oxidation

It was established that ferritin from pea seed is composed of 26.5 and 28.0 kDa subunits, but the relationship between the two subunits is unclear. The present study by both MALDI-TOF-MS and MS/MS indicated that the 28.0 kDa subunit is distinct from the 26.5 kDa subunit although they might share high homology in amino acid sequence, a result suggesting that pea seed ferritin is encoded by at least two genes. This result is not consistent with previous proposal that the 28.0 kDa subunit is converted into the 26.5 kDa subunit upon cleavage of its N-terminal sequence by free radical. Also, present results indicated that pea seed ferritin contains two different kinds of ferroxidase centers located in the 28.0 and 26.5 kDa subunits, respectively. This is an exception among all known ferritins. Therefore, it is of special interest to know the role of the two subunits in iron oxidative deposition. Spectrophotometric titration and stopped flow results indicated that 48 ferrous ions can be bound and oxidized by oxygen at the ferroxidase sites, demonstrating that all of the ferroxidase sites are active and involved in fast Fe(II) oxidation. However, unlike H and L subunits in horse spleen ferritin (HoSF), both the 28.0 and 26.5 subunits lack cooperation in iron turnover into the inner cavity of pea seed ferritin.