Free radicals, lipid peroxidation, and Parkinson's disease

The Etiopathogenesis of Parkinson Disease and Suggestions for Future Research. Part II

We are at a critical juncture in our knowledge of the etiology and pathogenesis of Parkinson disease (PD). It is clear that PD is not a single entity simply resulting from a dopaminergic deficit; rather it is most likely caused by a combination of genetic and environmental factors. Although there is extensive new information on the etiology and pathogenesis of PD, which may advance its treatment, new syntheses of this information are needed. The second part of this two-part, state-of-the-art review by leaders in PD research critically examines the research field to identify areas for which new knowledge and ideas might be helpful for treatment purposes. Topics reviewed in Part II are genetics, animal models, and oxidative stress.

Mitochondrial dysfunction, peroxidation damage and changes in glutathione metabolism in PARK6

Oxidative stress and protein aggregation are biochemical hallmarks of Parkinson's disease (PD), a frequent sporadic late-onset degenerative disorder particularly of dopaminergic neurons in the substantia nigra, resulting in impaired spontaneous movement. PARK6 is a rare autosomal-recessively inherited disorder, mimicking the clinical picture of PD with earlier onset and slower progression. Genetic data demonstrated PARK6 to be caused by mutations in the protein PINK1, which is localized to mitochondria and has a serine-threonine kinase domain. To study the effect of PINK1 mutations on oxidative stress, we used primary fibroblasts and immortalized lymphoblasts from three patients homozygous for G309D-PINK1. Oxidative stress was evident from increases in lipid peroxidation and in antioxidant defenses by mitochondrial superoxide dismutase and glutathione. Elevated levels of glutathione reductase and glutathione-S-transferase were also observed. As a putative cause of oxidation, a mild decrease in complex I activity and a trend to superoxide elevation were detectable. These data indicate that PINK1 function is critical to prevent oxidative damage and that peripheral cells may be useful for studies of progression and therapy of PARK6.

Oxidative stress and genetics in the pathogenesis of Parkinson's disease

Parkinson's Disease (PD) is the second most common chronic neurodegenerative disease characterized by the progressive loss of dopamine neurons, leading to rigidity, slowness of movement, rest tremor, gait disturbances, and imbalance. Although there is effective symptomatic treatment for PD, there is no proven preventative or regenerative therapy. The etiology of this disorder remains unknown. Recent genetic studies have identified mutations in alpha-synuclein as a rare cause of autosomal dominant familial PD and mutations in parkin as a cause of autosomal recessive familial PD. The more common sporadic form of PD is thought to be due to oxidative stress and derangements in mitochondrial complex I activity. Understanding the mechanism by which familial linked mutations and oxidative stress cause PD has tremendous potential for unraveling the mechanisms of dopamine cell death in PD. In this article, we review recent advances in the understanding of the role of genetics and oxidative stress in the pathogenesis of PD.

Peripheral iron metabolism in patients with Parkinson's disease

To elucidate the possible role of peripheral metabolism of iron in the risk for developing Parkinson's disease (PD), we compared serum levels of iron, transferrin and ferritin, and 24-h iron excretion in urine after a single intramuscular dose of 1 mg/kg desferrioxamine, in 68 PD patients and their spouses as the control group. All these values did not differ significantly between the groups, they were not influenced by antiparkinsonian therapy, and they did not correlate with age, age at onset and duration of the disease, scores of the Unified PD Rating Scale or the Hoehn and Yahr staging in the PD group, with the exception of the 24-h urinary iron excretion with the duration of the disease (r = 0.32, p < 0.05). These results suggest that peripheral metabolism of iron is apparently unrelated to the risk of developing PD.

Increased iron in the substantia nigra compacta of the MPTP-lesioned hemiparkinsonian African green monkey: evidence from proton microprobe elemental microanalysis

The association of free radicals and particularly free iron in the pathogenesis of idiopathic Parkinson's disease and MPTP-induced parkinsonism remains controversial. Whereas the actual cause of dopamine cell death in the substantia nigra compacta (SNc) remains unknown, disturbances in lipid peroxidation and subsequent mitochondrial and cell membrane disruption has been demonstrated. In a genetically susceptible host, abnormal elimination of oxygen and trace metal free radicals may further damage dopamine cells. Using a unilaterally MPTP-treated African Green monkey, which showed obvious contralateral hemiparkinsonism, the total free iron concentration was measured. Iron, Fe2+ and Fe3+, but not other trace elements, was significantly elevated in the SNc compared with the opposite unlesioned side, which was similar to separate control animals. Iron content in the SNc, periaqueductal gray area, and crus cerebri was 228-270 ppm. Normal control SNc was 285 (+/- 59) ppm, whereas iron levels of 532 (+/- 151) ppm were found in the MPTP-lesioned SNc. These animals were drug naive and not on long-term levodopa maintenance. Proton microprobe elemental analysis was matched against adjacent immunocytochemically stained tissue slices to ensure the cells studied were in the SNc. Iron was found not only in the degenerating dopamine cells themselves but also in the surrounding matrix and glial cells. Whether free iron that is not bound to neuromelanin is responsible for dopamine cell death as suggested by these experiments remains to be proved.

Serum levels of ascorbic acid (vitamin C) in patients with Parkinson's disease

To elucidate the possible role of vitamin C in the risk for developing Parkinson's disease (PD), we compared serum levels of ascorbic acid (vitamin C), measured by a fluorometric method, of 63 PD patients using their spouses as the control group. The serum levels of vitamin C did not differ significantly between the groups (47.13 +/- 0.89 micrograms/ml for PD and 47.60 +/- 0.60 micrograms/ml for controls). There was no influence of antiparkinsonian therapy on vitamin C. Serum levels of vitamin C did not correlate with age, age at onset and duration of the disease, scores of the Unified PD Rating Scale or the Hoehn and Yahr staging in the PD group. These results suggest that serum vitamin C concentrations are apparently unrelated to the risk of developing PD.

Serum levels of beta-carotene and other carotenoids in Parkinson's disease

To elucidate the possible role of carotenoids in the risk for developing Parkinson's disease (PD), we compared serum levels of beta-carotene, alpha-carotene and lycopene, measured by high performance liquid chromatography, of 61 PD patients using their spouses as the control group. The serum levels of these 3 carotenoids did not differ significantly between PD patients and control groups. There was no influence of antiparkinsonian therapy on serum carotenoids levels, and these did not correlate with age, age at onset, scores of the Unified Parkinson Disease Rating Scale or the Hoehn and Yahr staging in the PD group. These results show that serum carotenoids concentrations are apparently unrelated to the risk for developing PD.

Serum lipid peroxides in patients with Parkinson's disease

To elucidate whether high serum lipid peroxidation rates may increase the risk of developing Parkinson's disease (PD), we assessed serum levels of malondialdehyde (MDA), an intermediate in lipid peroxidation processes, in 37 PD patients, with their spouses as the control group. Serum MDA levels did not differ significantly between these two groups (8.7 +/- 0.51 and 8.8 +/- 0.48 nmol/ml, resp.), and were not influenced by antiparkinsonian therapy in the PD patients. Serum MDA levels were inversely correlated with age and age at onset (P less than 0.01) in the PD group, but they were not correlated with disease duration, Unified Parkinson's Disease Rating Scale scores or Hoehn and Yahr staging. In the control group there was no correlation between serum MDA and age. These results suggest that, although serum levels of lipid peroxides were similar in both the PD and control groups, high serum lipid peroxidation rates might constitute a risk factor for younger onset of PD in predisposed individuals.

Glutathione in Parkinson's disease: a link between oxidative stress and mitochondrial damage?

Several links exist between the two mechanisms of neuronal degeneration (i.e., oxygen radical production and mitochondrial damage) proposed to have a role in Parkinson's disease. Indeed, mitochondria are critical targets for the toxic injury induced by oxygen radicals, and experimental evidence suggests that mitochondrial damage may cause an increased generation of oxygen radicals. A potentially important link between these two mechanisms of neurodegeneration is glutathione. Because of the scavenging activity of glutathione against accumulation of oxygen radicals, its decrease in the brains of parkinsonian patients has been interpreted as a sign of oxidative stress; however, this change may also result from or lead to mitochondrial damage. It is conceivable therefore that regardless of whether oxidative stress or mitochondrial damage represents the initial insult, these toxic mechanisms may both contribute to neuronal degeneration via changes in glutathione levels.

Environment, genetics and idiopathic Parkinson's disease

Since Idiopathic Parkinson's disease (IPD) was first described more than 170 years ago, there have been major advances in the understanding of the etiology of the disease as well as in its treatment. This article will review current knowledge concerning the role of the environment, genetic hypotheses and the aging factor in the etiology of IPD and proposes a complex interaction involving all these factors. Hypotheses regarding mitochondrial inhibition and free radical generation in IPD are discussed in relation to the mechanism of action of neurotoxins known to produce parkinsonian syndromes.

The MPTP model: versatile contributions to the treatment of idiopathic Parkinson's disease

In human and subhuman primates, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces irreversible clinical, biochemical and neuropathological alterations highly reminiscent of those observed in Parkinson's disease. The MPTP model has provided the best available tool to date for the assessment of efficacy and side-effects of symptomatic treatments of Parkinson's disease. In addition, the mechanism of action of MPTP has offered a basis for the development of novel therapeutic strategies aimed at the prevention of Parkinson's disease.

Increased erythrocyte susceptibility to lipid peroxidation in human Parkinson's disease

Recent evidence suggests that among the factors that lead to neurodegenerative changes in Parkinson's disease are stimulation of lipid peroxidation and deficiency of glutathione and glutathione peroxidase in substantia nigra. We have investigated the effect of neurodegenerative changes on plasma and erythrocytes of patients with Parkinson's disease and compared the results with those of age-matched controls. Both plasma lipid peroxide levels and erythrocyte susceptibility to lipid peroxidation were significantly increased in Parkinson's disease. Erythrocyte fragility tests revealed that in 35% of the patients there was increased fragility. In addition, erythrocyte catalase activities were not changed whereas glutathione levels and glutathione peroxidase activities were decreased in Parkinson's disease. Our results suggest that erythrocyte membrane integrity may be impaired in Parkinson's disease.

Prevention of progression of Parkinson's disease with antioxidative therapy

1. Oxidative mechanisms in dopaminergic neurons may contribute to cell death and the progression of Parkinson's Disease. 2. The free radical auto-toxicity concept has scientific evidence to support it. 3. Clinical trials are underway to assess the protective effect of augmenting the free radical scavenging system with vitamin E and inhibiting catecholamine oxidation with deprenyl.

Antioxidant therapy in Parkinson's disease

It is postulated that endogenous oxidative mechanisms are a major factor in the continuing death of dopaminergic neurons and the progression of Parkinson's disease. Scientific evidence in support of, and negating, the free radical auto-toxicity and dopamine toxicity concepts is reviewed. There is conflicting evidence whether free radicals are involved in the toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and attempts to prevent the toxicity of MPTP with antioxidant therapy have had variable results. The oxidation of dopamine by monoamine oxidase produces toxic metabolites however animal studies with high dose longterm levodopa and MPTP have failed to show clear evidence for autoxidation. Firm supportive evidence is obtained from the monoamine oxidase B inhibitor experience which demonstrated a block of the toxicity of MPTP in animals and probable prolongation of the course of human Parkinson's disease. The scientific data available is inconclusive but there is significant hope of retarding progressive catecholaminergic neuron degenerative changes by augmenting the free radical scavenging system with antioxidants (such as Vitamin E) and slowing catecholamine oxidation by monoamine oxidase B inhibition. Careful clinical trials with these agents must be performed.