Free Thiol of Transthyretin in Human Plasma Most Accessible to Modification/Oxidation

Free-thiol(s) in proteins, especially, when located on the surface of the molecule, are susceptible to oxidation/modification, which may cause loss of function or an alteration in the ternary structure. This suggests that the status of thiol group(s) of cysteine residue(s) in a protein, i.e., free-thiol versus an oxidized/modified form, in vivo, could reflect the physiological state of the molecule with respect to susceptibility to oxidative stress. To address this issue, we established an efficient method for isolating proteins that contain free thiol groups from a complex mixture, which permits the amount of free-thiol form(s) to modified/oxidized forms to be estimated. Albumin, which accounts for 55% of the total plasma proteins and has such a free thiol and has been reported to scavenge various reactive oxygen species (ROS) in vivo. The developed method was used to isolate the free form of albumin from fresh plasma. However, contrary to our expectations, transthyretin (TTR), which also has a single free thiol, was found to be the major protein that was the most susceptible to modification/oxidation. In addition, the free-thiol form could be separated from oxidized or modified molecules, permitting the relative abundance of the free-thiol form to be estimated. The findings show that the levels of the free-thiol form of TTR in plasma was significantly lowered after a hydrogen peroxide treatment, even at low concentrations (0.1 mM), suggesting that TTR could be a useful biomarker for monitoring a ROS imbalance in relation to various oxidative stress conditions.