NADPH-dependent Reductases Involved in the Detoxification of Reactive Carbonyls in Plants

Abstract

Reactive carbonyls, especially ␣,␤-unsaturated carbonyls produced through lipid peroxidation, damage biomolecules such as proteins and nucleotides; elimination of these carbonyls is therefore essential for maintaining cellular homeostasis. In this study, we focused on an NADPH-dependent detoxification of reactive carbonyls in plants and explored the enzyme system involved in this detoxification process. Using acrolein (CH 2 ‫؍‬ CHCHO) as a model ␣,␤-unsaturated carbonyl, we purified a predominant NADPH-dependent acrolein-reducing enzyme from cucumber leaves, and we identified the enzyme as an alkenal/one oxidoreductase (AOR) catalyzing reduction of an ␣,␤-unsaturated bond. Cloning of cDNA encoding AORs revealed that cucumber contains two distinct AORs, chloroplastic AOR and cytosolic AOR. Homologs of cucumber AORs were found among various plant species, including Arabidopsis, and we confirmed that a homolog of Arabidopsis (At1g23740) also had AOR activity. Phylogenetic analysis showed that these AORs belong to a novel class of AORs. They preferentially reduced ␣,␤-unsaturated ketones rather than ␣,␤-unsaturated aldehydes. Furthermore, we selected candidates of other classes of enzymes involved in NADPH-dependent reduction of carbonyls based on the bioinformatic information, and we found that an aldo-keto reductase (At2g37770) and aldehyde reductases (At1g54870 and At3g04000) were implicated in the reduction of an aldehyde group of saturated aldehydes and methylglyoxal as well as ␣,␤-unsaturated aldehydes in chloroplasts. These results suggest that different classes of NADPH-dependent reductases cooperatively contribute to the detoxification of reactive carbonyls.When plants are subjected to abiotic and/or biotic stresses, oxidative stress often results; this leads to the production of reactive oxygen species, which damage biomolecules such as proteins and lipids. More than half of the fatty acids found in the membranes of chloroplasts and mitochondria, two of the most highly oxidative organelles, are linoleic and linolenic acid. Because linoleic and linolenic acid are sources of many short chain carbonyls through their peroxidation (1), biomolecules in both types of organelles are challenged by the toxicity of reactive compounds, including ␣,␤-unsaturated carbonyls, which are involved in the pathophysiological effects associated with oxidative stress in cells and tissues (2). In fact, ␣,␤-unsaturated carbonyls from peroxidized polyunsaturated fatty acids cause loss of functions in mitochondria (3); chloroplasts have recently been shown to be a major production center of reactive aldehydes (4), and photosynthetic functions are highly sensitive to ␣,␤-unsaturated carbonyls (5-7).The high reactivity of ␣,␤-unsaturated carbonyls is due to the ability of their ␣,␤-unsaturated bonds to form Michael adducts with thiol and amino groups in biomolecules; in the case of ␣,␤-unsaturated aldehydes, the aldehyde group also contributes to reactivity through the formation of Schiff base adducts with amino groups. Thus the targe...

Glutathionylation and reduction of methacrolein in tomato plants account for its absorption from the vapor phase

“…The genes for AOR and AKR form a family, and each member shows distinct substrate specificity (Yamauchi et al, 2011;Saito et al, 2013). As far as we know, an enzyme with high specificity for MACR has not been reported so far; therefore, we do not know if there is a reductase specific to MACR.…”

“…In cucumber (Cucumis sativus) and Arabidopsis, NADPHdependent AORs are involved in the reduction of the double bond (Yamauchi et al, 2011). Another pathway for detoxification of reactive carbonyl species reduces the carbonyl to alcohol and is catalyzed by AKRs that also prefer NADPH as the reducing cofactor (Yamauchi et al, 2011;Matsui et al, 2012).…”

Identification of (Z)-3:(E)-2-Hexenal Isomerases Essential to the Production of the Leaf Aldehyde in Plants

“…(Z)-3 and (E)-2, based on the position of the unsaturated bond in their structures. (Z)-3-Hexenal and (E)-2-hexenal are subsequently reduced to alcohols by aldehyde reductases, aldo/keto reductases (11), and alcohol dehydrogenases (12). Alcohol forms of GLVs are further converted to ester forms by a BAHD acyltransferase (2).…”

The importance of lipoxygenase control in the production of green leaf volatiles by lipase-dependent and independent pathways

“…This implied that there must be an NADPH-specific aldo-keto reductase with a relatively strict substrate specificity to C6 aldehydes. In Arabidopsis, several enzymes belonging to the aldo-keto reductase superfamily or NADB_ Rossmann (short-chain dehydrogenase/reductase) superfamily are involved in detoxification of reactive carbonyls having α,β-unsaturated carbonyl moieties, such as (E)-2-hexenal (Yamauchi et al 2011). Again, it is uncertain whether the Arabidopsis reductases examined so far can reduce (Z)-3-hexenal to (Z)-3-hexen-1-ol.…”