Human serum albumin-mercurial species interactions

Binding of metal ions to the heteroatomic sites of proteins is undoubtedly fundamental to their observed physiological effects. In this paper, the interactions of inorganic mercury (Hg2+), methylmercury (MeHg+), ethylmercury (EtHg+), and phenylmercury (PhHg+) with human serum albumin (HSA) were studied from the electrophoretic behaviors, stoichiometry, thermodynamics, and kinetics by using a new hybrid technique, capillary electrophoresis on-line coupled with electrothermal atomic absorption spectrometry (CE-ETAAS), together with the consequent structural information from circular dichroism and Raman spectroscopy. The stoichiometry (mercurial species to HSA) for the interactions of Hg2+, MeHg+, EtHg+, and PhHg+ with HSA was found to be 6:1, 4:1, 4:1, and 3:1, respectively. Two types of binding sites in HSA were observed for the binding of mercurial species with the orders of magnitude of binding constants of 10(7) and 10(6) L mol-1, respectively, showing strong affinity of mercurial species for HSA. The interactions of mercurial species with both types of binding sites in HSA are exothermic and thermodynamically favorable and are both enthalpically and entropically driven. The binding of mercurial species to HSA follows the first-order kinetics for mercurial species and zero-order kinetics for HSA with the apparent activation energy of 57-59 kJ mol-1. Among the four mercurial species examined, only Hg2+ induces the secondary structure transition of HSA. Mercury-HSA adducts are formed mainly through metal-sulfur binding with participation of C=O and/or C-N groups of amino acid residues in HSA molecules. The present work represents the most comprehensive study on the interactions between various mercurial species with HSA and provides new evidence for and insights into the interactions of mercurial species with HSA for further understanding of the toxicological effects of mercurial species.