A proteomic approach was applied to mitochondrial protein isolated from the livers of rats fed a combination high-fat and ethanol diet to identify proteins modified by 4-hydroxynonenal (4-HNE). Using this approach, the endoplasmic reticulum chaperone, protein disulfide isomerase (PDI), which participates in the maturation of newly synthesized proteins through promoting correct disulfide formation, was consistently found to be modified by 4-HNE. Further mass spectral analysis of PDI isolated from the animals revealed modification of an active site Cys residue thought to be involved in client protein binding. To test the hypothesis that 4-HNE inhibits the chaperone, purified bovine PDI was treated with concentrations of 4-HNE ranging from 20 to 200 microM (10-100-fold molar excess aldehyde), resulting in 14-56% inhibition, respectively. Similar treatments with the lipid peroxidation products acrolein (ACR) and 4-oxononenal (4-ONE) resulted in 60 and 100% inhibition, respectively, suggesting inactivation of the chaperone via Cys modification. Thiol sensitivity was confirmed through concentration-dependent inhibition of PDI by the Cys modifier N-ethylmaleimide (NEM). While some degree of sensitivity to these lipid aldehydes is suggested by the data, when compared to inactivation of other proteins by 4-HNE, PDI has demonstrated a relative resistance. It was also observed that physiologic (e.g., 4 mM) concentrations of GSH were capable of removing the 4-HNE adducts, likely serving as a protective mechanism against inactivation by 4-HNE and other lipid peroxidation products. However, because an active site Cys was found to be modified by 4-HNE on PDI in vivo, it is possible that the protective effect of GSH on the chaperone decreases under conditions of sustained oxidative stress, such as during chronic alcohol consumption, as GSH is depleted. The data presented here thus suggest potential impairment of an important molecular chaperone during oxidative stress.