Enzymes in Parkinson's disease
Lauro Galzigna,Dept.Biochemistry,University of Padua
Parkinson's disease(PD) is a chronic progressive disorder of the central nervous system(CNS) clinically recognized by slowness and poverty of movements,muscular rigidity and tremor.
The general features of PD such as tremor,rigidity and akinesia,with asymmetry of symptoms and signs,response to levodopa,midbrain atrophy with striatonigral degeneration,death of selected neuron populations,appearance of Lewy bodies derived from degenerating neurites,mitochondrial dysfunction,autosomal dominant inheritance, have been extensively reviewed(Lang & Lozano,1998).
The disease is mostly idiopathic and may be post-traumatic,postencephalytic or drug-induced as in phenothiazine-treated patients.The biochemical changes occur initially in the substantia nigra(SN) and corpus striatum with loss of melanin and dopamine from the SN.In normal conditions SN dopamine is localized within the cell body,while in the striatum it is found at the synaptic end of the axons.Electrical stimulation of SN releases dopamine in the striatum and a SN damage decreases the striatal dopamine.In the striatum of PD brain there are increased concentrations of acetylcholine and acetylcholinesterase,while intrastriatal injections of acetylcholine increase its electrical activity and injections of dopamine decrease it.Anticholinesterase drugs aggravate PD.
A plausible hypothesis on the pathogenesis of PD is that its biochemical correlates are a decreased dopaminergic and an increased cholinergic activity.
In previous papers(Galzigna et al.,1996) we have shown that phenothiazines are peroxidase substrates and their main oxidized product is a phenothiazine sulfoxide that can replace hydrogen peroxide in the peroxidase-catalyzed reaction(Galzigna et al.,1997).
A dopamine peroxidase activity was isolated in the midbrain of adult male rats(Galzigna et al.,1999).The enzyme was inhibited by carnosine and other antioxidants,while the reaction needs hydrogen peroxide and its product is dopaminochrome.
It is known that in normal human brain the highest peroxidase activity is localized in SN and this activity decreases in PD together with catalase.Caudate and putamen show similar changes(Ambani et al.,1975).In autopsied brain of PD patients, glutathione peroxidase was found to be lowered in SN and other areas which were also deficient of reduced glutathione(GSH) and this was indicated as a factor contributing to the nigral dopamine neurons loss(Kish et al.,1985).The depletion of GSH observed in SN cells of PD patients with no changes in oxidized glutathione(GSSG) may be due to GSH efflux and increased GSH-GSSG conversion due to increased extracellular hydrogen peroxide formation(Sian et al.,1994).The elevation of iron in SN of PD patients may justify increased oxidative stress and neurotoxicity documented by the above said cohort of changes,altough it is difficult to assess whether they are primary or secondary to other neurotoxic events(Youdim et al.,1993).
Glutathione peroxidase and superoxide dismutase also increase in serum of PD patients and this was interpreted as a result of the oxidative stress in the initial stages of the disease(Kalra et al.,1992).In adult rats, peroxidase-positive astroglia increasing with age was found in SN and the iron-mediated peroxidase activity may be a promoter of dopamine bioactivation to a neurotoxic free radical intermediate which predisposes the senescent brain to PD(Schipper et al.,1998).In any case,the whole theory of the oxidative stress is highly hypothetical since it is unclear whether it is a cause or a consequence of the disease(Hirsch,1993).Transplantation of embryonic nigral tissue ameliorates the functional deficiencies of PD and it was observed that caspase inhibition reduces apoptosis in transplants,with an increased survival of dopaminergic neurons grafted to hemiparkinsonian rats(Schierle et al.,1999).The involvement of proteases in PD has been thus advocated although their role and importance are still unclear(McGeer,1998).In any case some derangement in cathepsin proteinases and their endogenous inhibitors was observed in the parkinsonian dementia of Guam(Ii et al.,1993).
Tumor necrosis factor(TNF),a cytokine with neuroimmune effect has specific receptors on lymphocytes and other cells and it has been found that T-lymphocytes of PD patients have increased levels of TNF-alpha receptor(Bongioanni et al.,1997).In the cerebrospinal fluid(CSF) of PD patients,increased levels of cytokines were measured and interpreted as a compensatory brain response to the neurodegenerative process or,alternatively,as a neurodegeneration trigger(Mogi et al.,1996).
The disappearance of dopaminergic neurons,as well as that of melanin from the SN is a characteristic sign of PD and it is believed that oxidative stress,together with inflammatory processes and apoptosis may concur to this phenomenon(Marsden et al.,1998).Apoptosis has been in fact observed as a result of intrastriatal injections of the neurotoxin 6-hydroxydopamine in animal models of PD(Burke & Kholodilov,1998),while,quite generally,apoptosis is considered as one of the mechanisms of neurodegenerative diseases,probably depending on a genetic factor(Drago et al.,1998).Other authors,on the contrary,claim that apoptosis is not a mechanism of cell death in PD(Kosel et al.,1997).
The function of neuromelanin produced by dopaminergic cells is still an enigma but some think that it may be an apoptosis inducer(Offen et al.,1997) and this confirms the idea that apoptosis is not a cause of PD.
The real cause of dopamine cells death is therefore a mistery,despite hypotheses concerning a mitochondrial oxidative dysfunction,excess of free radicals,deficient neurotrophic factors which by the way were used to treat idiopathic PD(Temlett,1996).Such factors are upregulated by MAO inhibitors,and also influenced by GSH,SOD,catalase and BCL2,a protein which protects against oxidative stress and apoptosis(Jenner & Olanov,1996).In cultured neurons, dopamine induces in any case apoptosis which is inhibited by antioxidants(Ziv et al.,1994),in accord with observations on the relationship between apoptosis and free radical production(Seaton et al.,1997).Also an anti-apoptotic protein was found to be significantly raised in basal ganglia of PD patients and yet,as for the oxidative stress,nobody knows whether apoptosis is a cause or consequence of neurodegeneration(Marshall et al.,1997).Inhibitors of proteases of the caspase family block apoptosis and increase survival of transplanted dopaminergic neurons in hemiparkinsonian rats(Schierle et al.,1999),but again it has been suggested that apoptosis detected in postmortem midbrain samples of PD patients may just be a perimortem effect(Kingsbury et al.,1998).
In conclusion,in PD there are decreases in peroxidase,melanin formation,bioreductive defences,and increases in iron,oxidative stress,possibly apoptosis somehow related to raised cytokine levels.The changes considered to be typical of PD are the loss of dopaminergic neurons and melanin,but if melanin and also dopamine are apoptosis inducers,this rules out the role of the latter as a primary cause of PD:
It is therefore plausible that apoptosis be a consequence of PD and an imbalance between oxidative and reductive steps is at the origin of the impairment of the melanin pathway biosynthesis.
We have shown that dopaminochrome is a cell killer for different in vitro cultured cells(Galzigna et al.,1999) and an accumulation of this compound depending on different factors(e.g. increased hydrogen peroxide levels and decreased bioreductive defences) may be the most probable pathogenetic factor for PD.Dopaminochrome can be also generated by peroxidase which utilizes hydrogen peroxide or,in alternative,phenothiazine sulfoxides as oxidants.The latter is the case of phenothiazine therapy which,as already pointed out,can induce PD.An increased acetylcholine may either influence the dopaminochrome formation by complexing it, as shown long time ago(Galzigna,1970), or the formation of melanin.
In a very interesting paper,Grisham et al.(1987) reported the purification of a canine nigrostriatal peroxidase located on the outer edge of the mitochondrial membrane.The enzyme is possibly responsible for nigral neuromelanogenesis and possesses potent cytotoxic properties.The latter are inhibited by physiological levels of dopamine and, since higher levels of hydrogen peroxide lead to an increased oxidation of dopamine,the final result of the dopamine disappearance is evidently the destruction of the dopaminergic neurons.
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