Removal of ferritin‐bound iron by DL‐dihydrolipoate and DL‐dihydrolipoamide

Abstract

The naturally occurring dithiols DL-dihydrolipoate and DL-dihydrolipoamide were tested for their ability in the removal of ferritin-bound iron. Both compounds remove the iron stored inside the protein by complexing it in the ferric form. The iron can be reduced to the ferrous form by excess dithiol, but this is not necessary for complete removal. Reaction is complete in few hours and, at molar ratios of chelator to metal higher than 10, more than 60% of the ferritin-bound iron was removed. The amount of iron stored in the ferritin molecule does not affect the rate and the yield of the removal reaction.The iron-removing ability of DL-dihydrolipoate was found to be identical to that of an equimolar solution of sodium dithionite, and to be pH-dependent.Results are discussed in terms of the molecular architecture of ferritin and of the chelators, and their possible physiological relevance is pointed out.Iron storage and mobilisation in mammalian cells have drawn the attention of many investigators from different fields. Ferritin plays a central role in iron metabolism, so that studies of this protein have been particularly intensive. Highly detailed structural information about this molecule is available [l], its response to metabolic and pathological changes in the iron concentration has been extensively studied [2, 31 and a large number of compounds have been tested for their ability in the removal of the iron stored inside the ferritin protein shell [4 -61. Removal of ferritin-bound iron is a prominent topic from the point of view of either biochemists and clinicians, since it relates to the metabolic fate of iron and to the treatment of the iron overload, which is consequent to, among others, the heavy transfusional therapy of some blood diseases. The most widely used chelators are of bacterial origin, are fairly expensive and require parentheral administration [7].In the course of other studies we noticed the ease of formation and the stability of complexes of iron(1II) with the physiological dithiols DL-dihydrolipoate and uL-dihydrolipoamidc [8, 91. The size of these two compounds, their amphiphilic nature, thcir rcducing properties and their natural occurrence in mammalian tissues, where they are formed from the corresponding oxidized compounds, led us to an in vitro investigation of the reaction between them and horse spleen ferritin, which gave very encouraging preliminary results [I 01.In the present paper we wish to report a more extensive investigation of this subject.C'orrc,spondc~ic.nc~~ lu

Iron transport and storage

“…Iron release is rather slow both for a number of biological chelators [298-3011 and with synthetic chelators of Fe(II1) [300, 3021 or with microbial siderophores [303 -3051. However much more rapid rates were observed for the chelation of ferritin iron under argon by dihydrolipoate and dihydrolipoamide [306]. In general much more rapid rates of iron release are observed with reducing agents such as dithionite, dihydroflavins or thioglycollate [307 -31 11 in the presence of a$-bipyridyl.…”

Iron-based ferritin nanocore as a contrast agent

“…Unbound iron was removed by buffer exchange using an Amicon centrifugal filter device ͑100 kD MWCO; Millipore, Billerica, MA͒. Following iron loading and buffer exchange, the encapsulated iron content of the protein was determined by conversion of the encapsulated ferric complex to the ferrous form and quantification with bathophenanthroline disulfonic acid, disodium salt hydrate at 538 nm, as previously described by Bonomi et al 26 with slight modification. The stoichiometric loading amount was calculated to be ϳ7200 Fe/24-mer.…”

Iron‐binding Proteins

“…Uptake occurs most readily in ferritin molecules approximately half full, probably because these possess the largest core surface area. Release of iron in vitro requires reducing agents such as dithionite or reduced flavines (92,93), or free radicals (94), and is enhanced at low pH (95). Iron release in vivo probably occurs in lysosomes, where the acid pH will be advantageous.…”