Lipid Peroxidation Products Inhibit Dopamine Catabolism Yielding Aberrant Levels of a Reactive Intermediate

Abstract

Recent work indicates that oxidative stress is a factor in Parkinson's disease (PD); however, it is unknown how this condition causes selective dopaminergic cell death. The neurotransmitter dopamine (DA) has been implicated as an endogenous neurotoxin to explain the selective neurodegeneration. DA undergoes catabolism by monoamine oxidase (MAO) to the reactive intermediate 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is further oxidized to 3,4-dihydroxyphenylacetic (DOPAC) acid via mitochondrial aldehyde dehydrogenase (ALDH). Previous studies found DOPAL to be more toxic than DA, and the major lipid peroxidation products, that is, 4-hydroxynonenal (4HNE) and malondialdehyde (MDA), potently inhibit ALDH. The hypothesis of this work is that lipid peroxidation products inhibit DOPAL oxidation, yielding aberrant levels of the reactive aldehyde intermediate. Treatment of striatal synaptosomes with 2-100 microM 4HNE or 2-50 microM MDA impaired DOPAL oxidation, resulting in elevated [DOPAL]. The aberrant concentration of DOPAL yielded an increase in protein modification by the DA-derived aldehyde, evident via staining of proteins with nitroblue tetrazolium (NBT). Pretreatment of synaptosomes with an MAO inhibitor significantly decreased NBT staining. On the basis of NBT staining, the order of protein reactivity for DA and metabolites was found to be DOPAL>>DOPAC>DA. Mass spectrometric analysis of a model peptide reacted with DOPAL revealed the adduct to be a Schiff base product. In summary, these data demonstrate the sensitivity of DA catabolism to the lipid peroxidation products 4HNE and MDA even at low, physiologic levels and suggest a mechanistic link between oxidative stress and generation of aberrant levels of an endogenous and protein reactive dopaminergic toxin relevant to PD.

Oxidation of 3,4-Dihydroxyphenylacetaldehyde, a Toxic Dopaminergic Metabolite, to a Semiquinone Radical and an ortho-Quinone

“…Alternatively, DA may undergo monoamine oxidase-catalyzed oxidative deamination to produce DOPAL, which can be further metabolized by aldehyde dehydrogenase to DOPAC (16,40). Previously, we showed that lipid peroxidation, which is closely associated with oxidative stress, can inhibit aldehyde dehydrogenase causing elevated intracellular DOPAL levels (17,24). DOPAL is highly toxic and is hypothesized to be a causative factor in PD (15,22).…”

“…However, these mechanisms do not fully account for the high levels of toxicity associated with DOPAL. Therefore, we and others have hypothesized that DOPAL may be capable of undergoing oxidation to a quinone (16,17,20,22).…”

“…However, these mechanisms do not fully account for the high levels of toxicity associated with DOPAL. Therefore, we and others have hypothesized that DOPAL may be capable of undergoing oxidation to a quinone (16,17,20,22).Oxidation of DOPAL to an ortho-quinone is predicted to have deleterious effects on cells and could result in additional mechanisms of toxicity (16,17,20,22). Work from our laboratory suggests that protein reactivity associated with DOPAL is partially mediated via catechol oxidation (18).…”

“…Physiological levels of DOPAL in the substantia nigra are ϳ2 M; levels as low as 6 M can exert significant toxicity (16). Reactivity with proteins, presumably via Schiff base formation, is an important mechanism of toxicity for DOPAL (17,18). Protein modification can result in inhibition of enzyme activity and loss of function for cellular proteins.…”

“…Oxidation of DOPAL to an ortho-quinone is predicted to have deleterious effects on cells and could result in additional mechanisms of toxicity (16,17,20,22). Work from our laboratory suggests that protein reactivity associated with DOPAL is partially mediated via catechol oxidation (18).…”