ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
P196. Interaction of Eu(III) Derivatives with Human Serum Albumin
L. Trynda-Lemiesz, R. Janicki, A. Mondry
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
e-mail: ltl@wchuwr.pl
Serum albumins are the most abundant proteins in blood plasma, accounting for about 60% of the total protein.
Human serum albumin (HSA) binds and transports many exogenous and endogenous ligands, including fatty
acids, metal ions, and pharmaceuticals HSA consists of three structurally homologous domains (I, II, and III)
that assemble to form a heartshaped molecule. Solution of the X-ray crystallographic structure of HSA facilitated
the location of the two major drug binding sites, site I and site II proposed originally by Sudlow et al., in
subdomains IIA and IIIA of the protein, respectively [1, 2].
The lanthanide-based pharmaceuticals to date are either non-specific agents or have suffered from toxicity or
efficacy problems. Recent developments in the fields of coordination chemistry and biotechnology have taken
great strides towards tissue targeted diagnostic and therapeutic agents. The interactions of lanthanide complexes
with blood constituents, particularly with serum albumin indicates the importance of the molecular shape of the
complexes.
Studies of gadolinium(III) complexes with serum albumin [3] suggest that the rate of water exchange in the
complex may be slowed upon binding to HSA. This would presumably be the result of secondary interactions
between protein and the chelate that hinder access of the bulk water to coordinated water site in the complex.
Human Serum Albumin Molecular structure of Eu(III)–EDTMP–carbonate complex
In the present work a mechanism of the interaction of Eu(III)–EDTMP complex (where EDTMP is
ethylenediaminetetra(methylenephosphonate) ligand) with HSA has been considered. The identification of
binding sites and the nature of forces involved in the interaction were studied using fluorescence and CD
spectroscopic techniques. The decrease of relative fluorescence intensity of the Eu(III)–EDTMP–bound HSA
suggests that perturbation around the Trp 214 residue takes place. This was confirmed by the destabilization of
the warfarin binding site located in subdomain IIA. CD spectroscopic results showed a discernible reduction in
the affinity of albumin for bilirubin upon Eu(III)–EDTMP binding. These results may indicate that one of the
binding sites of the complex is subdomain IIA.
Recently it was shown that at physiological pH a partial hydrolysis of the Eu(III)–EDTMP complex occurs [4] as
well as the equilibrium between species of [Eu(EDTMP)(H2O)2] 5– and [Eu(EDTMP)(H2O)(OH)] 6– exists [5]. It
was also revealed that the replacement of water molecules and/or hydroxy groups by a carbonate anion in the
Eu(III)–EDTMP complex at pH 7.5 results in the formation of thermodynamically stable and kinetically inert
[Eu(EDTMP)(CO3)] 7– species of which crystal structure has been determined [5]. Probably formation of
hydrogen bond between HSA and inner-sphere water molecules of Eu(III) ion in the energetically unfavorable
[Eu(EDTMP)(H2O)2] 5– species is a reason of a weak hydrogen interaction between this species and protein. The
lack of fluorescence intensity changes in the phosphate buffered solution between the spectra of HSA and
Eu(III)–EDTMP–HSA may indicate the replacement of inner-sphere water molecules onto phosphate anion.
The presented investigations may be helpful in understanding of the uptake mechanism of the 153 Sm(III)–
EDTMP complex by metastatic bones and may provide some indications for future ligand design for therapeutic
complexes with lanthanide radionuclides.
References:
[1] D.C. Carter, J.X. Ho, Adv. Protein Chem., 45, 152 (1994).
[2] X.M. He, D.C Carter, Nature, 358, 209 (1992).
[3] S. Aime, M. Botta, M. Fasano, S.G. Crich, E. Terreno, J. Biol. Inorg. Chem, 1, 312 (1996).
[4] G.C. de Witt, P.M. May, J. Webb, G. Hefter, BioMetals, 9, 351 (1996).
[5] A. Mondry, R. Janicki, Dalton Trans., 4702 (2006).
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