CNS dopamine oxidation and catechol-O-methyltransferase: Importance in the etiology, pharmacotherapy, and dietary prevention of Parkinson's disease

In this article, a particular emphasis has been placed on the conceptual development and understanding of the unique pathogenic changes that are indigenous to the striatal dopaminergic neurons as an important etiological factor in human Parkinson's disease (PD) as well as on the understanding of their clinical implications. Specifically, I have discussed the etiological roles of central nervous system dopamine oxidation in PD, along with a critical review of the available evidence in support of the proposed hypotheses. The chemically-reactive dopamine quinone/semiquinone intermediates are known to be highly neurotoxic and potentially genotoxic. There is considerable evidence for the suggestion that the long-term use of levodopa accelerates the progression of PD. In comparison, centrally-acting non-catechol dopamine receptor agonists would be an execellent alternative to levodopa for the treatment of PD (particularly for late-stage PD) because these agents would not undergo redox cycling to cause oxidative neuronal damage. Catechol-O-methyltransferase (COMT)-mediated methylation metabolism of catecholamine neurotransmitters is a crucial first-line detoxification pathway, and its role in the causation and prevention of PD is also discussed. On the basis of the modulation of COMT-mediated methylation of catecholamines, it is mechanistically explained that hyperhomocysteinemia would be a pathogenic factor in PD whereas vitamins B6, B12, and folate would be a protective factor. Lastly, according to the mechanistic understanding developed here, a novel dietary strategy is proposed that is specifically tailored toward lowering the risk of human PD, which includes eating a nutritionally-balanced diet that contains adequate (but not excessive) amounts of fruits and vegetables, along with adequate dietary supplementation of S-adenosyl-L-methionine, vitamins C, B6, B12, and folate. It is believed that these conceptual developments would also aid in our better understanding of other age-related neurodegenerative disorders, such as Alzeihmer's and Huntington's diseases.