Benomyl, aldehyde dehydrogenase, DOPAL, and the catecholaldehyde hypothesis for the pathogenesis of Parkinson's disease.

The dopamine metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL) is detoxified mainly by aldehyde dehydrogenase (ALDH). We find that the fungicide benomyl potently and rapidly inhibits ALDH and builds up DOPAL in vivo in mouse striatum and in vitro in PC12 cells and human cultured fibroblasts and glial cells. The in vivo results resemble those noted previously with knockouts of the genes encoding ALDH1A1 and 2, a mouse model of aging-related Parkinson's disease (PD). Exposure to pesticides that inhibit ALDH may therefore increase PD risk via DOPAL buildup. This study lends support to the "catecholaldehyde hypothesis" that the autotoxic dopamine metabolite DOPAL plays a pathogenic role in PD.

Pesticides have long been implicated in the pathogenesis of Parkinson’s disease (PD).[1−3] A recent epidemiologic study noted increased PD risk from occupational or residential exposure to the fungicide benomyl.[4,5] Benomyl undergoes bioactivation to S-methyl N-butylthiocarbamate sulfoxide (MBT-SO), a potent aldehyde dehydrogenase (ALDH) inhibitor,[6,7] and ALDH is the main enzyme metabolizing the toxic dopamine metabolite, 3,4-dihydroxyphenylacetaldehyde (DOPAL). Thus, a potential mechanism for increased PD risk from benomyl exposure is ALDH inhibition and consequent DOPAL build-up,[4] in line with the “catecholaldehyde hypothesis” (Figure 1).[8−13]

Figure 1

Catecholaldehyde hypothesis for PD. The toxic dopamine (DA) metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL) is formed from mitochondrial monoamine oxidase (MAO) acting on cytoplasmic DA. DOPAL cytotoxicity occurs via oxidative injury and protein cross-linking. DOPAL is detoxified by aldehyde dehydrogenase (ALDH) to form 3,4-dihydroxyphenylacetic acid (DOPAC). An alternative metabolite is 3,4-dihydroxyphenylethanol (DOPET).

In this study, we tested whether benomyl produces a neurochemical pattern indicating ALDH inhibition, in vivo in mouse striatum and in vitro in rat pheochromocytoma PC12 cells and in human cultured fibroblasts and glial cells.

Mice received intraperitoneally (ip) administered benomyl at 40 mg/kg and were sacrificed 2 h later and the brains recovered. Frozen striatum samples were homogenized in 20:80 0.2 M phosphoric acid/0.2 M acetic acid and the supernate transferred to plastic cryotubes and stored at −80 °C until assayed by batch alumina extraction followed by liquid chromatography with serial electrochemical detection[14,15] (Figure 2A). Three and only three of the seven catechols analyzed showed significant changes relative to controls without benomyl (Figure 2B). DOPAL increased by 3.1-fold and DOPET by 2.5-fold, while DOPAC decreased. ALDH inhibition was evident by the decrease in DOPAC with respect to elevations of both DOPAL and DOPET. The tissue concentration ratio of DOPAC/(DOPAL + DOPET) was therefore used as a neurochemical index of ALDH activity. The magnitude of the benomyl-induced decrease in the striatal DOPAC/(DOPAL + DOPET) ratio (Figure 2C1 and D1) was not greatly changed when benomyl was coadministered with reserpine (40 mg/kg) and L-DOPA (20 mg/kg) (Figure 2C2 and D2) and closely resembled that reported previously in Aldh1a1–/– × Aldh2–/– knockout mice (Figure 2C3 and D3), an animal model of aging-related PD.[16]

Figure 2

Effects of benomyl (40 mg/kg, ip, 2 h after treatment) and ALDH −/– gene knockout on DA-derived catechols in mouse striatum. (A) Chromatogram of extracted catechols from a control mouse. (B) Catechol levels on a log scale showing benomyl-induced increase in DOPAL and DOPET and decrease in DOPAC (mean ± SEM). (C1–C3) DOPAL, DOPET, and DOPAC levels on a linear scale and D1–D3 the same data expressed as DOPAC/(DOPAL + DOPET) ratios. C1 and D1: 2 h after benomyl. C2 and D2: 2 h after benomyl with reserpine and L-DOPA. C3 and D3: ALDH −/– mice data from Wey et al.[16] Levels are relative to no benomyl (C1 and C2) or ALDH −/– (C3). Different from control (mean ± SEM, n = 5–6), **p < 0.01, ***p < 0.001, and ****p < 0.0001.

To supplement the in vivo data, three types of cells were used to determine effects of benomyl in vitro on contents of catechols. Nonadherent rat PC12 cells[17,18] were obtained from ATCC (Manassas, VA). The PC12 cells were cultured in F12 media containing 15% human serum (HS) plus 2.5% fetal calf serum (FCS) and pretreated for 24 h with 10 μM tolcapone to block catechol-O-methyltransferase.[13] Human fibroblasts were from a skin biopsy of a PD patient. The fibroblasts were cultured in minimal essential medium (MEM)-alpha containing 10% FCS and penicillin/streptomycin. Human glioblastoma cells were from ATCC. The glial cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) containing 10% FCS.

PC12 cells (500,000/well) were placed in 12 well plates and incubated for 24 h at 37 °C. The medium was then changed to F12 containing benomyl (0, 5, 10, 50, 100, 500, and 1000 nM). The medium was removed and the cells collected at 180 min, transferred into Eppendorf sample tubes, centrifuged, and resuspended in 400 μL of 20:80 0.2 M phosphoric acid/0.2 M acetic acid, frozen in dry ice, and stored at −80 °C before thawing for catechol assays. Human fibroblasts (50,000/well) were cultured for 3 days and glioblastoma cells (100,000/well) for 2 days in 12 well plates at 37 °C in a 5% CO2 incubator. Then, the medium was replaced with FCS-free MEM-alpha for fibroblasts and DMEM for glia containing no benomyl (control) or 1000 nM benomyl and preincubated for 10 min before adding 3 μM DOPAL to determine its metabolism as the substrate. The samples were collected at 10, 20, 30, 60, 120, and 180 min. The medium was removed, and cells were washed with 1× PBS and then scraped into 400 μL of 20:80 phosphoric acid/acetic acid as described above, frozen in dry ice, and kept at −80 °C before thawing for catechol assays.

In all three cell types, benomyl greatly decreased the DOPAC/(DOPAL + DOPET) ratios (Figure 3). The concentration for half-maximal effect with PC12 cells was about 50 nM, based on an analysis of the cells or the media at 180 min (Figure 3A). Time-series studies established the rapid action of 1000 nM benomyl in both human fibroblasts and glia (Figure 3B and C). Thus, in the cells as in the mouse striatum, the principal catechols affected were DOPAL and DOPET increasing and DOPAC decreasing, as expected for ALDH inhibition.

Figure 3

Effects of benomyl on DOPAC/(DOPAL + DOPET) ratios in three cell types. (A) PC12 cells and media at 180 min as a function of benomyl concentration (0–1000 nM). (B and C) Human cultured fibroblasts and glial cells at 0 nM (control) or 1000 nM benomyl as a function of time after adding DOPAL as the substrate (mean ± SEM, n = 3–5).

ALDH inhibitors are of both pharmacological and toxicological interest and include several pesticides.[19,20] The mouse and cell systems described here are potential in vivo and in vitro models to assay pesticides, environmental chemicals, and pharmaceuticals as candidate contributors to PD by disrupting DOPAL detoxification (Figure 1).

An alternative to the catecholaldehyde hypothesis of PD that still involves ALDH inhibition is trans-4-hydroxy-2-nonenal as the neurotoxicant formed on membrane lipid peroxidation and serving as an ALDH inhibitor for both its own detoxification and that of DOPAL.[21−23] The catecholaldehyde hypothesis predicts straightforwardly that inhibition of MAO should attenuate pesticide-evoked cytotoxicity. This has been demonstrated for the insecticide rotenone with pathophysiological mechanisms involving mitochondrial dysfunction and oxidative stress[1−3] and for benomyl as an ALDH inhibitor.[4]