Binding sites of retinol and retinoic acid with serum albumins

Retinoids are effectively transported in the bloodstream via serum albumins. We report the complexation of bovine serum albumin (BSA) with retinol and retinoic acid at physiological conditions, using constant protein concentration and various retinoid contents. FTIR, CD and fluorescence spectroscopic methods and molecular modeling were used to analyze retinoid binding site, the binding constant and the effects of complexation on BSA stability and secondary structure. Structural analysis showed that retinoids bind BSA via hydrophilic and hydrophobic interactions with overall binding constants of KRet–BSA = 5.3 (±0.8) × 106 M−1 and KRetac–BSA = 2.3 (±0.4) × 106 M−1. The number of bound retinoid molecules (n) was 1.20 (±0.2) for retinol and 1.8 (±0.3) for retinoic acid. Molecular modeling showed the participation of several amino acids in retinoid–BSA complexes stabilized by H-bonding network. The retinoid binding altered BSA conformation with a major reduction of α-helix from 61% (free BSA) to 36% (retinol–BSA) and 26% (retinoic acid–BSA) with an increase in turn and random coil structures indicating a partial protein unfolding. The results indicate that serum albumins are capable of transporting retinoids in vitro and in vivo.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The binding sites of retinol and retinoic acid were located on BSA and HSA. ► Retinoids bind serum albumins via several amino acids with hydrophobic and hydrophilic contacts. ► Stronger complexes formed with BSA leading to protein destabilization. ► The results support the effective transportation of retinoids via serum albumins in vitro. ► Strong acid-BSA binding reduces the isomerization of cis-retinoic acid to trans-retinoic acid.