In vitro plasma protein binding and aqueous aggregation behavior of astaxanthin dilysinate tetrahydrochloride

TitleIn vitro plasma protein binding and aqueous aggregation behavior of astaxanthin dilysinate tetrahydrochloride
Publication TypeJournal Article
Year of Publication2004
AuthorsZsila, F, Fitos, I, Bikádi, Z, Simonyi, M, Jackson, HL, Lockwood, SF
JournalBioorganic and Medicinal Chemistry Letters
Pagination5357 - 5366
Date Published2004
KeywordsAGP, Astaxanthin, Astaxanthin di-l-lysinate tetrahydrochloride, Card-pack aggregation, Carotenoid derivatives, Chiral complexation, Circular dichroism spectroscopy, HSA, Human serum albumin, Induced chirality, α 1-Acid glycoprotein

Distinct monomeric and chirally-complexed binding of astaxanthin dilysinate tetrahydrochloride to human serum albumin. The tetrahydrochloride salt of astaxanthin di-l-lysinate (lys 2AST) is a highly water-dispersible astaxanthin-amino acid conjugate, with an aqueous dispersibility of ≥181.6 mg/mL. The statistical mixture of stereoisomers has been well characterized as an aqueous-phase superoxide anion scavenger, effective at micromolar (μM) concentrations. In the current study, the aqueous aggregation behavior and in vitro plasma protein binding [with fatty-acid-free human serum albumin (HSA) and α 1-acid glycoprotein (AGP)] were investigated with a suite of techniques, including circular dichroism (CD) and UV-vis spectroscopy, ultrafiltration, competitive ligand displacement, and fluorescence quenching. Induced CD bands obtained in Ringer buffer solution of HSA demonstrated high affinity monomeric binding of the compound at low ligand per protein (L/P) ratios (in aqueous solution alone the carotenoid molecules formed card-pack aggregates). The binding constant (∼10 6 M -1) and the binding stoichiometry (∼0.2 per albumin molecule) were calculated from CD titration data. CD displacement and ultrafiltration experiments performed with marker ligands of HSA indicated that the ligand binding occurred at a site distinct from the main drug binding sites of HSA (i.e., Sites I and II). At intermediate L/P ratios, both monomeric and aggregated ('chirally complexed') binding occurred simultaneously at distinct sites of the protein. At high L/P ratios, chiral complexation predominantly occurred on the asymmetric protein template. The tentative location of the chirally-complexed aggregation on the HSA template was identified as the large interdomain cleft of HSA, where carotenoid derivatives have been found to bind previously. Only weak binding to AGP was observed. These results suggest that parenteral use of this highly potent, water-dispersible astaxanthin-amino acid conjugate will result in plasma protein association, and plasma protein binding at sites unlikely to displace fatty acids and drugs bound at well-characterized binding sites on the albumin molecule. © 2004 Elsevier Ltd. All rights reserved.