Formation of a bovine serum albumin diligand complex with rutin for the suppression of heme toxicity

Serum albumin binds avidly to heme to form heme-serum albumin complex and can protect against the potentially toxic effects of heme. Rutin is a glycoside of the bioflavonoid quercetin with various protective effects due to its antioxidant ability. Clarification of the interaction mechanisms between serum albumin and bioactive components (such as heme and flavonoid) is important to develop effective carriers for encapsulation of heme and suppression of its toxicity. In this study, bindings of bovine serum albumin (BSA) to heme and/or rutin were investigated by experimental and molecular docking techniques. The fluorescence of BSA was quenched by both heme and rutin in static mode (i.e. formation of BSA-monoligand complexes), which was confirmed by Stern-Volmer calculations. Although heme showed higher affinity to BSA than rutin, the interactions of both components with BSA did locate within subdomain IIA (site I). BSA-diligand complexes were successfully formed after the simultaneous addition of heme and rutin. Bioactive rutin in the BSA-diligand complex still kept strong free radical scavenging activity compared to free rutin or BSA-monoligand complex. Hydrogen peroxide (H₂O₂)-induced heme degradation and free iron release was inhibited upon BSA binding and further decreased in BSA-diligand complexes. Consistently, the cytotoxicity of heme and oxidative stress in endothelial cells was decreased in the BSA-diligand complexes relative to those of heme or BSA-heme complex, where the co-presence of rutin played an important role. These results suggest the possibility and advantage of developing BSA-based carriers for the suppression of heme toxicity in their biomedical applications.