Antioxidative Function and Substrate Specificity of NAD(P)H- dependent Alkenal/one Oxidoreductase

There are several known routes for the metabolic detoxication of α,β-unsaturated aldehydes and ketones, including conjugation to glutathione and reduction and oxidation of the aldehyde to an alcohol and a carboxylic acid, respectively. In this study, we describe a fourth class of detoxication that involves the reduction of the α,β-carbon=carbon double bond to a single bond. This reaction is catalyzed by NAD(P)H-dependent alkenal/one oxidoreductase (AO), an enzyme heretofore known as leukotriene B4 12-hydroxydehydrogenase, 15-oxoprostaglandin 13-reductase, and dithiolethione-inducible gene-1. AO is shown to effectively reduce cytotoxic lipid peroxidation products such as 4-hydroxy-2-nonenal (HNE) (k cat = 4.0 × 103 min−1;k cat/K m = 3.3 × 107 min−1 m −1) and acrolein (k cat = 2.2 × 102min−1; k cat/K m= 1.5 × 106 min−1 m −1) and common industrial compounds such as ethyl vinyl ketone (k cat = 9.6 × 103 min−1;k cat/K m = 8.8 × 107 min−1 m −1) and 15-oxoprostaglandin E1 (k cat = 2.4 × 103 min−1;k cat/K m = 2.4 × 109 min−1 m −1). Furthermore, transfection of human embryonic kidney cells with a rat liver AO expression vector protected these cells from challenge with HNE. The concentration of HNE at which 50% of the cells were killed after 24 h increased from ∼15 μm in control cells to ∼70 μm in AO-transfected cells. Overexpression of AO also completely abolished protein alkylation by HNE at all concentrations tested (up to 30 μm). Thus, we describe a novel antioxidative activity of a previously characterized bioactive lipid-metabolizing enzyme that could prove to be therapeutically or prophylactically useful due to its high catalytic rate and inducibility.