Physiologically relevant plasma d,l-homocysteine concentrations mobilize Cd from human serum albumin

• Stability of a human serum albumin (HSA)-Cd (1:1) complex was studied using a hyphenated analytical approach (SEC-FAAS).
• Addition of d,l-homocysteine (0.5 mM) to the mobile phase abstracted Cd from its binding sites on HSA.
• Since the blood plasma concentration of d,l-homocysteine can reach 0.5 mM in certain patients (homocystinurics) and since a thiol-based carrier exists on the surface of hepatocytes, the uptake of Cd into the liver may contribute to the disease progression.
• SEC-FAAS approach emerges as a useful tool to gain insight into processes which govern the translocation of toxicologically relevant metal species to their mammalian target organs.

Although low-level chronic exposure of humans to cadmium (Cd2+) can result in a variety of adverse health effects, little is known about the role that its interactions with plasma proteins and small molecular weight (SMW) ligands in the bloodstream may play in delivering this metal to its target organs. To gain insight, a Cd-human serum albumin (HSA) 1:1 (molar ratio) complex was analyzed by size exclusion chromatography (SEC) coupled on-line to a flame atomic absorption spectrometer (FAAS). Using a phosphate buffered saline (PBS)-buffer mobile phase, the stability of the Cd-HSA complex was investigated in the presence of 2.0 mM of SMW ligands, including taurine, acetaminophen, l-methionine, l-cysteine (Cys), d,l-homocysteine (hCys) or l-cysteine methyl-ester (Cys-Me). While taurine, acetaminophen and l-methionine did not affect its integrity, Cys, hCys and Cys-Me completely abstracted Cd from HSA. Subsequent investigations into the effect of 1.5, 1.0 and 0.5 mM Cys and hCys on the integrity of the Cd-HSA complex revealed clear differences with regard to the nature of the eluting SMW-Cd species between these structurally related endogenous thiols. Interestingly, the Cd-specific chromatograms that were obtained for 0.5 mM hCys revealed the elution of an apparent mixture of the parent Cd-HSA complex with a significant contribution of a structurally uncharacterized CdxhCysy species. Since this hCys concentration is encountered in blood plasma of hyperhomocysteinemia patients and since previous studies by others have revealed that a SH-containing carrier mediates the uptake of Cd into hepatocytes, our results suggest that plasma hCys may play a role in the toxicologically relevant translocation of Cd from the bloodstream to mammalian target organs.