Parkinson’s disease: oxidative stress and therapeutic approaches
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- Surendran, S. & Rajasankar, S. Neurol Sci (2010) 31: 531. doi:10.1007/s10072-010-0245-1
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Parkinson’s disease (PD) is a neurodegenerative disorder, caused by reduced levels of catecholamines and oxidative stress. Symptoms seen in the disease include tremor, rigidity, bradykinesia and postural disability. Oxidative stress plays a key role in neurodegeneration and motor abnormalities seen in PD. Altered levels of the protein caused by these changes lead to defective ubiquitin–proteasome pathway. Neurodegeneration seen in PD and Canavan disease has a common mechanism. Recent studies suggest that herbal medicines can improve molecular changes and motor functions seen in PD.
KeywordsParkinson’s diseaseAntioxidantWithania somniferaNeurodegenerationCanavan disease
Parkinson’s disease (PD) is a neurological disorder and approximately one million people are affected by the disease in North America. In 1817, James Parkinson first described the disease in his “An essay on the Shaking Palsy” . Subsequently, the disease was termed as Parkinson’s disease. Patients with PD show symptoms such as tremor, rigidity (stiffness in the muscle), bradykinesia (slowness of movement) and impaired balance [1–3]. Some patients with PD also show secondary symptoms, which include depression, pain, abnormal facial expression due to stiffness in the facial muscle and constipation. Substantia nigra pars compacta provides dopaminergic input to the striatum and these are the major nuclei of the basal ganglia. Although loss of dopaminergic neurons is mainly seen in the substantia nigra pars compacta [2, 3], the striatum is also involved in PD [4, 5]. Recent studies suggest that oxidative damage plays a major role in PD.
The brain contains a high amount of phospholipids and polyunsaturated free fatty acids. These are vulnerable to oxidants. Oxidative stress is one of the contributors in the loss of dopaminergic neurons in PD [4, 5]. Tyrosine is converted into 3,4-dihydroxyphenylalanine (l-dopa) by tyrosine hydroxylase and subsequently to dopamine (DA). Environmental factors affect mitochondrial enzymes such as complex I, resulting in decreased energy metabolism . Mitochondria generate adenosine triphosphate (ATP). Down regulation of the complex I activity impedes proteasomal functions through ATP depletion . In addition, oxidative stress leads to accumulation of unwanted proteins. These changes lead to the deficiency of ubiquitin–proteasome. Thus, PD patients substantia nigra has defective 20S proteasome [7, 8] accompanied by the accumulation of misfolded and unwanted proteins.
Antioxidants protect cells from oxidative damage caused by various factors including environmental factors. Antioxidants such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and reduced glutathione (GSH) protect cells during oxidative stress. Important antioxidant levels are found to be altered in PD. CAT that catalyzes the decomposition of hydrogen peroxide (H2O2) to water and oxygen (O2) is increased during oxidative damage. The antioxidant SOD (EC 184.108.40.206), which catalyzes the dismutation of superoxide into O2 and H2O2, is found to be increased in PD . GSH, an antioxidant that protects cells from free radical insult, is decreased in PD. GPx (EC 220.127.116.11), which reduces lipid hydroperoxidases to their corresponding alcohols and free H2O2 to water, is found to be decreased in PD . Antioxidant-containing therapies have been shown to be effective in the animal models with PD. Therefore, the present review was aimed at understanding the molecular changes in PD and possible therapeutical approaches in treating the oxidative damage and motor function in PD.
Clinical symptoms of the disease
The clinical features of the disease are tremor, rigidity, bradykinesia and postural disability. Tremor occurs in the hands, forearm, foot and fingers while the patients are at rest, but not while they are at work. Patients with PD have difficulty in walking and have a sensation of falling forward. Secondary symptoms include constipation, depression, dysphagia (difficulty in swallowing), dementia (loss of intellectual), micrographia (small cramped hand writing), anxiety and bradyphrenia (late response to questions). PD has been classified into different forms. A sporadic form of PD is seen in over 90% of all PD patients and the likely cause of the disease is environmental factors. The familial form of PD is associated with genetic defect and is seen in approximately 10% of all PD patients .
Patients with idiopathic PD do not develop all the symptoms seen in PD; however, their symptoms include bradykinesia, tremor, rigidity, reduced arm swinging while walking, intellectual problems, difficulty in talking and parkinsonian gait.
Parkinsonism is a condition similar to PD, characterized by tremor, hypokinesia (slow movement of muscle), rigidity, impaired speech and postural instability. A wide range of etiology is involved in parkinsonism, in addition to the cause seen in PD. Secondary cause of parkinsonism include meningitis, psychotic medications, encephalitis and trauma.
Catecholamine levels in the brain of patients with Parkinson’s disease
Reduction of catecholamine levels in the brain of patients with PD  (%)
3,4 Dihydroxyphenylacetic acid (DOPAC)
Homovanillic acid (HVA)
Dopamine levels in the basal ganglia of patients with Parkinson’s disease
Dopamine (ng/g tissue) 
External globus pallidus
490 ± 128
89 ± 20
Internal globus pallidus
75 ± 12
37 ± 9
4,833 ± 533
513 ± 164
6,475 ± 568
105 ± 28
Levels of dopamine, DOPAC and homovanillic acid in the corpus striatum and midbrain after Ws treatment 
Dopamine (μg/g tissue)
DOPAC (μg/g tissue)
Homovanillic acid (μg/g tissue)
12.36 ± 0.32
2.45 ± 0.07
1.53 ± 0.04
PD mouse not treated with Ws
4.13 ± 0.1
1.64 ± 0.03
0.62 ± 0.02
PD mouse treated with Ws
6.74 ± 0.15
1.85 ± 0.04
1.16 ± 0.03
27.6 ± 1.7
6.81 ± 0.51
1.51 ± 0.11
PD mouse not treated with Ws
7.34 ± 0.55
2.38 ± 0.17
0.62 ± 0.05
PD mouse treated with Ws
10.78 ± 0.78
3.42 ± 0.26
1.16 ± 0.08
12.57 ± 0.33
2.52 ± 0.06
1.46 ± 0.04
PD mouse not treated with Ws
3.54 ± 0.1
1.55 ± 0.04
0.58 ± 0.01
PD mouse treated with Ws
5.67 ± 0.18
1.76 ± 0.05
1.07 ± 0.03
Oxidative damage in the brain
Oxidative stress plays a key role in dopaminergic neuron death. Elevated levels of ROS is evident from increased lipid peroxidation and DNA damage in the substantia nigra and increased protein oxidation in the brain of PD . The details of the oxidative damage in PD is discussed below.
Lipid peroxidation in the brain of PD
Oxidative degradation of lipids is termed as lipid peroxidation. Lipid peroxidation is initiated by free radicals on the membrane lipids, which are capable of abstracting a hydrogen atom from the methylene group. The radical thus formed is stabilized by molecular rearrangement to produce conjugated diene, which easily reacts with an O2 molecule to give a peroxy radical. The peroxy radical can further abstract a hydrogen atom from another lipid molecule to form lipid hydroperoxides, which is known as the propagation stage of lipid peroxidation .
PD mouse treated with Ws
TBARS (nmoles/g tissue)
0.93 ± 0.07
3.15 ± 0.23
2.02 ± 0.15
SOD (U/mg protein)
1.31 ± 0.10
3.63 ± 0.27
2.17 ± 0.16
Catalase (U/mg protein)
3.08 ± 0.23
6.19 ± 0.41
4.57 ± 0.33
GSH (μg/g tissue)
0.39 ± 0.009
0.13 ± 0.003
0.25 ± 0.006
GPx (U/mg protein)
2.25 ± 0.06
1.43 ± 0.04
1.75 ± 0.01
SOD (U/mg protein)
0.77 ± 0.05
1.81 ± 0.12
1.19 ± 0.10
Catalase (U/mg protein)
0.48 ± 0.03
0.88 ± 0.05
0.61 ± 0.04
GPx (U/mg protein)
5.02 ± 0.39
4.05 ± 0.23
4.27 ± 0.35
Protein oxidation in the brain of PD
Autooxidation of DA produces H2O2 and DA quinine  leading to structural modifications of proteins. The levels of markers of oxidative damage to proteins, such as carbonyl modifications of soluble proteins, are elevated in the substantia nigra of PD [21, 22]. Protein oxidation is increased twofold in the substantia nigra of PD patients compared to normal subjects .
Abnormal levels of SOD in PD
CAT activity in PD brain
CAT requires nicotinamide adenine dinucleotide phosphate (NADPH) for regeneration from its inactive form . PD animal models showed elevated levels of CAT activity in the striatum and midbrain regions of the brain (Table 4) , probably to remove the toxic hydroxyl radicals in pathological condition.
GPx activity in the brain of PD
Reduced GSH activity in PD brain
Auto oxidation of DA produces H2O2 and depletion of the antioxidant, GSH [20, 27]. Subsequently, H2O2 is converted into hydroxyl radicals, which are highly reactive species capable of reacting with cellular macromolecules. Formation of hydroxyl radicals catalyzed by iron plays an important role in contributing to oxidative stress and dopaminergic neuron loss in PD [28, 29].
A reduced level of GSH was observed in the substantia nigra and corpus striatum of PD patients [30, 31] and also in animal models of PD (Table 4) suggesting that hydroxyl radical accumulation is increased in the brain of PD due to the downregulation of GSH.
Defective PARK proteins and mitochondrial dysfunction
Rare mutations in the glucocerebrosidase gene occur in almost 10% of PD patients in Japan. Mutations in the leucine-rich repeat kinase 2/LRRK2/PARK8 and α-synuclein/PARK1 genes lead to PD [32, 33]. Parkin/PARK2, DJ-1/PARK7, PTEN-induced putative kinase 1/PINK1/PARK6 and ATP13A2/PARK9 mutations cause recessive early-onset parkinsonism before 40 years of age. Parkin/PARK2-defective mice showed increased protein and lipid peroxidation . Parkin acts as a ubiquitin E3 ligase. In patients and mouse model with PD, mitochondrial electron transport complex I and IV activities are reduced [34, 35]. The PINK1/PARK6 is localized to the mitochondrial membrane . PINK1 mutations reduce complex I activity and increase oxidative damage , likely resulting in reduced ATP production. Oxidative stress relocalizes DJ-1/PARK7 protein from the nucleus to the mitochondrial matrix and intermembrane space [38, 39]. Leucine-rich repeat kinase 2/PARK8 is mainly present in the cytoplasm; a small amount of the protein is present in the outer mitochondrial membrane . High temperature requirement protein A2/PARK13 is a mitochondrial serine protease. HtrA2 knockout mice develop striatal degeneration . Mutations Gly399Ser or Ala141Ser causes mitochondrial swelling and decreased mitochondrial membrane potential . These studies suggest that PARK defect affect mitochondria function and the resulting unwanted proteins are accumulated due to the deficiency of ubiquitin–proteasome to contribute to neurodegeneration.
Defective mitochondria and ubiquitin–proteasome pathway in PD
Mitochondrial complex I activity is reduced in the brain of patients with PD  suggesting reduced electron transfer rate and increased oxidative damage in these patients’ brain. Magnetic resonance spectroscopy study of PD brain showed elevated cerebral lactate level [17, 44], suggesting altered aerobic metabolism due to mitochondrial dysfunction.
The α-synuclein protein is present in the axonal termini and is also a component of Lewy bodies. The protein contains an amino-terminal mitochondrial targeting sequence and overexpression can cause the protein to become localized to the mitochondria [32, 33]. Oxidative stress causes α-synuclein aggregation [7, 8, 19], which leads to the deficiency of the ubiquitin–proteasome.
The 20S proteasome contains immunoproteasome and constitutive proteasome. Immunoproteasome (i20S) has a proteolytic 20S core complex that contains LMP2 (β1i), MECL-1 (β2i) and LMP7 (β5i). Constitutive proteasome (c20S) has a proteolytic 20S core complex that contains β1, β2 and β5. LMP7 is important for the biogenesis of i20S [45, 46]. Downregulation of immunoproteasomes (i20S) likely upregulates the MHC peptides to remove the unwanted proteins present in the defective PD cells. Expression of MHC class I molecules are high in motoneurons and nigral dopaminergic neurons . In addition, the number of MHC class II (CR3/43)-positive activated microglia was also found to be increased in the brain of patients with PD . Oxidative damage-targeted cells having nonfunctional and unwanted proteins are presented by the MHC molecules to the T cells or killer cells, thus eliminating these cells.
Proteasomal function is affected in the substantia nigra pars compacta (SNc) in patients with sporadic PD. Deficiency of lysosomal enzymes caused by defective lysosome leads to protein accumulation and aggregation and neurodegeneration . Lysosome associated membrane protein 1 (LAMP1), cathepsin D (CatD) and heat shock protein 73 (HSP73) immunoreactivity were significantly decreased in alpha-synuclein-positive nigral neurons. 20S proteasome was also found to be reduced in alpha-synuclein-positive nigral neurons . These studies suggest that the ubiquitin–proteasome pathway as well as the lysosomal enzyme are defective in PD.
PD is characterized by the accumulation of ubiquitin in cytoplasmic inclusions known as Lewy bodies. Lewy bodies also contain elevated levels of a 19-kDa presynaptic vesicular protein, α-synuclein. Increase in α-synuclein levels form Lewy bodies in the midbrain, hippocampus and neocortex . α-synuclein is sensitive to oxidative stress . Oxidative stress induces polyubiquitin genes resulting in increased levels of monomeric ubiquitin in the cytoplasm to remove the damaged oxidized proteins [53, 54].
The familial form of PD genes are α-synuclein, parkin (ubiquitin ligase) and deubiquitinating enzyme, ubiquitin C-terminal hydrolase L1 (UCH-L1/PARK5) [55, 56]. UCH-L1 I93M mutation contributes to PD . α-synuclein is a fibrillar component of Lewy bodies. UCH-L1 forms dimer and ubiquitinate α-synuclein . Parkin is a ubiquitin ligase involved in dopaminergic neurodegeneration via the ubiquitin–proteasome pathway [59, 60]. Defect in the parkin gene caused by mutations does not lead to LB inclusion bodies in the brain samples of patients with PD , suggesting that parkin is not involved in LB formation. These studies suggest that when excess of unwanted and misfolded proteins are accumulated, the amount of ubiquitin–proteasome becomes deficient.
Salsolinol accumulation in PD brain
Salsolinol is synthesized from DA and acetaldehyde by salsolinol synthase. DA and pyruvic acid with the metabolite, salsolinol-1-carboxylic acid, also leads to the formation of solsolinol [61, 62]. Salsolinol was found to be distributed in various regions of the brain including the striatum , substantia nigra, frontal cortex , hypothalamus  and pituitary gland . While R-salsolinol was detected in the brain, R-salsolinol and S-salsolinol were observed in human plasma and urine . Salsolinol is converted into N-methyl-salsolinol by N-methyltransferase and subsequently catalyzed by amine oxidase to form 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion [62, 66]. Salsolinol inhibits mitochondrial complex II to induce neurodegeneration . Elevated levels of N-methylated derivates of salsolinol such as N-methyl salsolinol and its catalytic enzyme, N-methyltransferase, induces pathogenesis in PD [66, 68].
N-acetylaspartate levels in PD and Canavan disease
Canavan disease (CD) is a neurodegenerative disorder. Aspartoacylase gene mutations and elevated levels of NAA are seen in the brain of patients with CD. Clinical symptoms seen in the disease are complex, which include macrocephaly, hypotonia and mental retardation. Brain regions including striatum, cerebellum and spinal cord are involved in the disease. Patients with CD usually die in the first decade of life .
Ratio of NAA/Cr in the brain of patients with PD compared to control subjects
Patients with PD 
Percentage increase of NAA in the PD brain compared to the control brain (%↑)
1.55 ± 0.29
1.70 ± 0.37
1.28 ± 0.25
1.70 ± 0.37
1.32 ± 0.28
1.41 ± 0.28
Reactive astrocytes in PD
Astrocytes are important supporting cells for the transport of various factors including nutrients and amino acids to the neurons. Astrocytes converts glutamate to glutamine and it is transported to the neurons via presynaptic vesicles. After oxidative metabolism, neurons releasing carbon dioxide are maintained by astrocytes via acid–base balance and a defect in the astrocytes would lead to neurodegeneration. Classic reactive astrocytes were not observed in PD. Approximately, 45% of subcortical astrocytes in PD accumulated alpha-synuclein . Protoplasmic astrocytes (fibrous astrocytes) were expressed with parkin gene and accumulated abnormal proteins in PD . Presumably, expression of these proteins in the astrocytes in PD is likely to damage these cells. However, astrocyte numbers are found to be upregulated in PD . Astrocytes synthesize glutamate carboxypeptidase II (GCP II), an enzyme involved in the breakdown of N-acetyl aspartyl glutamate (NAAG) into NAA and glutamate. Upregulation of GCP II would result in increased production of NAA and glutamate . These changes are likely to contribute to neurodegeneration seen in PD. Upregulation of this pathway was also found during neurodegeneration in type 2 diabetes .
Possible treatments for oxidative stress in PD
Green tea polyphenols act as an antioxidant against free radicals such as superoxide anion [77–79], lipid free radicals and hydroxyl radicals [80, 81]. Oral administration of green tea polyphenols and flavonoid-related compounds in rat showed preventive effects on lipid peroxide formation [82, 83]. Soy isoflavone component genistein is a potential antioxidant [84, 85] that increases cellular reduced GSH . Green tea component epigallocatechin 3-gallate (EGCG) administration increased striatum DA, DOPAC and HVA levels. The compound also prevented elevated nitric oxide levels and dopaminergic neuronal loss . Inhibition of nitric oxide synthase by 7-nitroindazole prevented neurodegeneration in animal models of PD [88, 89]. Since nitric oxide toxicity is seen in other neurodegenerative diseases such as CD [4, 16] and ataxia telangiectasia , green tea components may also have potential therapeutic effect on these diseases.
Lentivirus-mediated expression of GPx protects against 6-hydroxydopa  suggesting that antioxidants can improve the catecholamine defect seen in PD. Overexpression of Cu, Zn-SOD and GPx protect against paraquat and maneb-induced PD phenotype in mice , further supporting that antioxidants can treat the physiological abnormalities seen in PD.
Withania somnifera/Ashwagandha is a plant that belongs to the Solanaceae family. The plant component has been used in Indian Ayurvedic medicine for thousands of years. The plant grows in various countries, which include India, Sri Lanka and South Africa. W. somnifera contains various alkaloids and steroidal lactones. Withanine is one of the alkaloids found in the plant and has several remedial effects. The leaves contain withanolides/steroidal lactones.
Recent studies have shown that W. somnifera extract had an effect on normalizing antioxidant SOD, GSH, GPx, CAT (Table 4) and also catecholamine, DA, DOPAC and HVA in the brain of PD mouse (Table 3) [5, 13]. In addition, this herbal drug could recover motor function  suggesting that normalizing antioxidants and catecholamine levels could result in recovery from physiological abnormalities seen in PD. From these studies, it is obvious that W. Somnifera extract can be used as a potential drug in treating oxidative stress and motor abnormalities in PD.
Vitamin E supplementation did not show any effect on PD pathophysiology . Coenzyme Q10 (CoQ10) is a naturally occurring antioxidant that affects mitochondrial depolarization and acts as an electron transporter for mitochondrial complexes I and II . CoQ10 levels are low in mitochondria, and the ratio of oxidized to reduced CoQ10 is greater in patients with PD. In the PD model, mouse and primate CoQ10 protects against oxidative stress and dopaminergic cell death [92, 97]. Combined therapy of CoQ10, 1,200 mg daily with α-tocopherol, slowed progression of the disease .
Rasagiline stabilized mitochondrial membrane potential  and showed some effect in the treatment of PD . Nonsteroidal anti-inflammatory drugs prevent the formation of hydroxyl radicals and production of elevated NO to protect dopaminergic neurons in PD .
Resveratrol induces genes associated with mitochondrial biogenesis and oxidative phosphorylation by activating peroxisome proliferator-activated receptor gamma coactivator . This drug protects dopaminergic neuron loss in PD model mouse .
Levodopa improves bradykinesia, rigidity, and gait in patients with PD [104, 105]. A glutamate antagonist, amantadine, could improve motor skills and dyskinesia in patients with PD . Treatment with DA analogs improves motor abnormalities. However, if the level of DA is high, patients suffer from motor and behavioral disturbances .
Parkinson’s disease is caused by reduced levels of catecholamines and oxidative stress. Oxidative stress has a key role in neurodegeneration and movement disorder seen in PD. Natural therapies could prevent catecholamine depletion, oxidative stress and improve motor abnormalities in animal models with PD. Neurodegeneration and movement mechanism seen in PD and CD have a common mechanism: the involvement of N-acetyl aspartic acid and nitric oxide toxicity.