4-Hydroxy-2-nonenal (HNE) is a toxic aldehyde generated during lipid peroxidation and has been implicated in a variety of pathological states associated with oxidative stress. Glutathione S-transferase (GST) A4-4 is recognized as one of the predominant enzymes responsible for the metabolism of HNE. However, substrate and product stereoselectivity remain to be fully explored. The results from a product formation assay indicate that hGSTA4-4 exhibits a modest preference for the biotransformation of S-HNE in the presence of both enantiomers. Liquid chromatography mass spectrometry analyses using the racemic and enantioisomeric HNE substrates explicitly demonstrate that hGSTA4-4 conjugates glutathione to both HNE enantiomers in a completely stereoselective manner that is not maintained in the spontaneous reaction. Compared with other hGST isoforms, hGSTA4-4 shows the highest degree of stereoselectivity. NMR experiments in combination with simulated annealing structure determinations enabled the determination of stereochemical configurations for the GSHNE diastereomers and are consistent with an hGSTA4-4-catalyzed nucleophilic attack that produces only the S-configuration at the site of conjugation, regardless of substrate chirality. In total these results indicate that hGSTA4-4 exhibits an intriguing combination of low substrate stereoselectivity with strict product stereoselectivity. This behavior allows for the detoxification of both HNE enantiomers while generating only a select set of GSHNE diastereomers with potential stereochemical implications concerning their effects and fates in biological tissues.Stereochemical configuration is a fundamental aspect of molecular structure, and the functional consequences of dynamic stereochemistry in biology are well established (1-6). Substrate stereoselectivity may occur in enzyme-mediated catalysis by virtue of the innate asymmetry of the active site. Product stereoselectivity may also arise when new chiral centers are introduced during an enzymatic reaction, because enzymes may specifically stabilize only one of the possible transition states for a given reaction. Stereoselective metabolism of both xenobiotic and endogenous molecules is a well recognized phenomenon that reflects this underlying chiral recognition process. To the extent that stereochemically distinct substrates and products have different biological effects, it is essential to define the stereochemical course of enzymatic reactions.Among the molecules generated endogenously from the degradation of polyunsaturated fatty acids during lipid peroxidation, the unsaturated aldehyde 4-hydroxy-2-nonenal (HNE) 2 has been implicated in a variety of pathological states associated with the deleterious consequences of oxidative stress, including atherosclerosis, diabetes, Alzheimer disease, and Parkinson disease (7-11). HNE has been proposed to exert a number of toxicological effects via its electrophilic ␣,-unsaturated carbonyl moiety that can react through additions with nucleophiles such as cysteine, histidine, a...