Relative Inhibitory Potency of Molinate and Metabolites with Aldehyde Dehydrogenase 2: Implications for the Mechanism of Enzyme Inhibition

Abstract

Molinate is a thiocarbamate herbicide used as a pre-emergent in rice patty fields. It has two predominant sulfoxidation metabolites, molinate sulfoxide and molinate sulfone. Previous work demonstrated an in vivo decrease in liver aldehyde dehydrogenase (ALDH) activity in rats treated with molinate and motor function deficits in dogs dosed chronically with this compound. ALDH is an enzyme important in the catabolism of many neurotransmitters, such as dopamine. Inhibition of this enzyme may lead to the accumulation of endogenous neurotoxic metabolites such as 3,4-dihydroxyphenylacetaldehyde (DOPAL), a dopamine metabolite, which may account for the observed neurotoxicity. In this study, the relative reactivity of molinate and both of its sulfoxidation metabolites towards ALDH were investigated, as well as the mechanism of inhibition. ALDH activity was monitored in two different model systems, human recombinant ALDH (hALDH2) and mouse striatal synaptosomes. Molinate sulfone was found to be the most potent ALDH inhibitor, compared to molinate and molinate sulfoxide. The reactivity of these three compounds was also assessed, using N-acetyl Cys, model peptides, and hALDH2. It was determined that molinate sulfone is capable of covalently modifying Cys residues, including catalytic Cys302 of ALDH, accounting for the observed enzyme inhibition.

Aldehyde Dehydrogenase Inhibitors: a Comprehensive Review of the Pharmacology, Mechanism of Action, Substrate Specificity, and Clinical Application

“…Both ALDH1A1 and ALDH2 have been implicated in dopamine metabolism. Inhibition of these isozymes leads to the accumulation of neurotoxic dopamine metabolites, such as 3,4-dihydroxyphenylacetaldehyde, that are thought to contribute to the pathogenesis of Parkinson's disease (Burke et al, 2003;Marchitti et al, 2007;Allen et al, 2010). Dopaminergic pathways are involved in the reinforcing effects of cocaine addiction.…”

“…ALDH inhibitory activity of the thiocarbamate pesticides was surmised from the observation that farmers working with these pesticides were rendered alcohol-sensitive (Hart and Faiman, 1995). Molinate and its sulfoxide and sulfone metabolites have been shown to inhibit human recombinant ALDH2 in vitro (Allen et al, 2010). This occurs through an irreversible modification of the active site cysteine residue (Cys302) and thiocarbamate formation (Fig.…”

Aldehyde dehydrogenase inhibition generates a reactive dopamine metabolite autotoxic to dopamine neurons

“…The thiocarbamate molinate was found to an inhibitor of ALDH, and more specifically, molinate and its sulfoxide and sulfone metabolites were found to inactivate human ALDH2 [23,24]. Enzyme inhibition was shown to result from covalent modification of the active site Cys302.…”

Proteomic analysis of hepatotoxicity induced by titanium nanoparticles in mouse liver

“…Overproduction of ROS breaks down the balance of oxidative/antioxidative system in the liver, resulting in the lipid peroxidation and hepatocyte apoptosis of mice, which may be closely associated with alterations of antioxidative enzyme levels. When antioxidative enzyme was inhibited, it led to enhanced oxidative stress, mitochondrial dysfunction, and toxicity [Allen et al, 2010]. In the present study, total activities of CAT and SOD were inhibited and the isozyme pattern for SOD was changed after exposure to TiO 2 NPs.…”