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Intracellular low molecular weight iron
N. DEIGHTON and R. C. HIDER
Depurtmetit of Pharmacy, Kitig :s C’ollege, Mutiresu Rotid,
London SW 3 hLX, U.K.
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Summary
A low molecular weight complex of an iron oligomer
( M , - 1000) has been isolated from rat livcr and characterized physically and chemically. The h.p.1.c.-purified
material contains the free amino acids glutamate and aspartate. Chemical techniques suggest the iron present in the factor is as iron(ll1) and is readily donated t o high-affinity
chelators such as the hydroxypyridinones and desferrioxamine.
The isolation utid characterization of u low molecirlar weight
iron complex f r o m rat liver
In biological systems the majority o f oxygen-derived free
radicals formed via metal complexes are produced by either
the Cu(l):Cu(II)or Fe(II):Fe(lll)redox couples.
The ability of metal complexes to generate hydroxyl radicals are dependent on both the stereochemical environment
and the binding affinity of the ligand for the metal (Singh &
Hider, 1Y88u, h).Furthermore. iron(l1)complexes tend to be
more efficient than those of iron(ll1). as the metal is in the
correct redox state to take part in thc reaction:
Fe(ll)+H,O,
-
F e ( l l I i + H O ~+HO.
For these reasons we argue that, to avoid toxicity, low
molecular mass ‘transit‘ species (Jacobs, 1 Y77) are probably
stable complexes of iron(lI1). These complexes must not,
however, be s o stable as to prevent metal transfer to ironrequiring metalloproteins.
A low molecular mass iron-containing fraction has been
isolated from rat liver using a sequence of chromatographic
procedures, including h.p.1.c. (Full details of purification and
chromatograms are available on request.) Thc molecular
mass of the species is estimated to be 1000- 1500. which is in
close agreement to previous reports (Mulligan et ul., 1986;
Jones Bi Grady. 1988).
Glu and Asp (as the free acids) were found to be associated with the metal at a Glu:Asp ratio of 7:2. Atomic
absorption spectroscopy reveals the liver concentration of
iron in this form is approximately 2 ,UM. Also associated with
the fraction are chromium (0.2 ,UM) and aluminium (0.4 ,uM).
The iron contained in the fraction is in the iron(ll1)state and
is rapidly chelated by 4-hydroxypyridinones, but less readily
by the hcxadentate chelator, dcsferrioxamine. Bathophenanthroline, an ironjll) chelator, only removes iron from the
fraction in the presence of dithionitc.
The U.V. spectrum of the fraction is similar to those of a
number of oligomeric iron cornpounds containing bridging
carboxylate functions (Fig. 1) prepared by Lippard ( 1Y 88).
The carboxylate-containing ligands may be simple anions
such as acetate, or more complex. such as alanine (Holt et al..
1 Y 74). Chromium(111) forms analogous compounds (Cotton
Bi Wilkinson, lY80).
A series o f compounds [Iron(111): acidic amino acids] have
been prepared for comparison with the purified liver fraction. These possess spectra similar to both the isolated fraction and the tri-iron(1II) mateiials of Lippard. In particular, a
compound, tentatively identified as Fe,O[Glu],, has been the
most extensively studied. Iron in this form is rapidly chelated
by 4-hydroxypyridinones, but more slowly by desferrioxamine. As is the case with the liver fraction, bathophenanthroline (in the absence of a reductant) will not remove
iron from the material. This synthetic material. like the fraction isolated from liver, readily donates iron to ferritin in the
presence o f phosphate. Preliminary e.s.r. studies have been
unable to distinguish the iron in the liver fraction from that in
the synthetic material.
It is also known that iron(II1)-carboxylate bonds are facile,
permitting rapid ligand exchange. The carboxylatesurrounded, 3-fold channels of ferritin are strikingly similar
in size and shape t o the tri-iron unit (Rice CI (11.. lY83). The
relative positions of the channel carboxylate functions suggest that ligand exchange with the Glu residues would be
possible. Indeed, such a mechanism of iron uptake is supported by thc finding that Fe-0-Fe moieties form o n the
surface o f ferritin (Chasteen et al., 1985; Yang el al., 1987).
Further physical and chemical characterization of both the
biological and synthetic materials is in progress.
We thank the M.R.C.f o r financial support
Chasteen. N . I).. Antanaitis, B. C . & Aisen, P. ( 1 0 x 5 ) .J. Iliol. C ’ h c r n .
260.2926-2929
Cotton. F. A. & Wilkinson. G. ( 19x0) Acli~riric~c~cl
Inorgtrriic~
C’hwni.s/ry. Fourth Edition, John Wiley and Sons. New York
Holt, E. M., Holt, S. L., Tucker, W. F., Asplund. R. 0.& Watson, K.
J . ( 1974) J . Arner. C‘hern. SOC. 9 6 , 2 6 2 1-2623
Jacobs. A . ( 1077) Aloorl 50. 333-439
Jones. R. L. & Grady. K.W. ( 19x8) pi~r.\(~/r(i/c.otnr~rrr,ric.tr/rorr
Lippard, S.J . ( I Y X X ) ilrrgcw. C’heni. 27. 334-30 I
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C‘orrc,cp/.s (Rice-Evans, C. & Halliwell, R., eds.), p. 0 1-90,
Richelieu Press
Yang, C. Y.. Meagher. A , , Huynh, B. H., Sayen. I). E. SC Theil.
E. C. ( 19x7) Hiochrmistry 26. 497-503
Received 24 November 1 V X X
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