Nature of non-transferrin-bound iron: studies on iron citrate complexes and thalassemic sera.
Despite its importance in iron-overload diseases, little is known about the composition of plasma non-transferrin-bound iron (NTBI). Using 30-kDa ultrafiltration, plasma from thalassemic patients consisted of both filterable and non-filterable NTBI, the filterable fraction representing less than 10% NTBI. Low filterability could result from protein binding or NTBI species exceeding 30 kDa. The properties of iron citrate and its interaction with albumin were therefore investigated, as these represent likely NTBI species. Iron permeated 5- or 12-kDa ultrafiltration units completely when complexes were freshly prepared and citrate exceeded iron by tenfold, whereas with 30-kDa ultrafiltration units, permeation approached 100% at all molar ratios. A g = 4.3 electron paramagnetic resonance signal, characteristic of mononuclear iron, was detectable only with iron-to-citrate ratios above 1:100. The ability of both desferrioxamine and 1,2-dimethyl-3-hydroxypyridin-4-one to chelate iron in iron citrate complexes also increased with increasing ratios of citrate to iron. Incremental molar excesses of citrate thus favour the progressive appearance of chelatable lower molecular weight iron oligomers, dimers and ultimately monomers. Filtration of iron citrate in the presence of albumin showed substantial binding to albumin across a wide range of iron-to-citrate ratios and also increased accessibility of iron to chelators, reflecting a shift towards smaller oligomeric species. However, in vitro experiments using immunodepletion or absorption of albumin to Cibacron blue-Sepharose indicate that iron is only loosely bound in iron citrate-albumin complexes and that NTBI is unlikely to be albumin-bound to any significant extent in thalassemic sera.
Evans
Robert W
RW
Metalloprotein Research Group, Nutritional Sciences Division, King's College London, New Hunt's House, London, SE1 1UL, UK. robert.evans@kcl.ac.uk
Rafique
Roozina
R
Zarea
Adel
A
Rapisarda
Chiara
C
Cammack
Richard
R
Evans
Patricia J
PJ
Porter
John B
JB
Hider
Robert C
RC
D002614:Chelating Agents / Q000378:metabolism; D005290:Ferric Compounds / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008670:Metals / Q000378:metabolism; D013789:Thalassemia / Q000378:metabolism*; D013816:Thermodynamics; D014462:Ultrafiltration
D002614:Chelating Agents; D005290:Ferric Compounds; D008670:Metals; C025314:ferric citrate