宮﨑 育子

The metallothionein (MT) family is a class of low molecular, intracellular, and cysteine-rich proteins with a high affinity for metals. Although the first of these proteins was discovered nearly 40 years ago, their functional significance remains obscure. Four major isoforms (MT-I, MT-II, MT-III, and MT-IV) have been identified in mammals. MT-I and MT-II are ubiquitously expressed in various organs including the brain, while expression of MT-III and MT-IV is restricted in specific organs. MT-III was detected predominantly in the brain, and characterized as a central nervous system-specific isomer. The role of MTs in the central nervous system has become an intense focus of scientific research. An isomer of MTs, MT-III, of particular interest, was originally discovered as a growth inhibitory factor, and has been found to be markedly reduced in the brain of patients with Alzheimer's disease and several other neurodegenerative diseases. MT-III fulfills unique biological roles in homeostasis of the central nervous system and in the etiology of neuropathological disorders.

Oxidative stress, including the reactive oxygen or nitrogen species generated in the enzymatical oxidationor auto-oxidation of an excess amount of dopamine, is thought to play an important role in dopaminergic neurotoxicity. Dopamine and its metabolites containing 2 hydroxyl residues exert cytotoxicityin dopaminergic neuronal cells, primarily due to the generation of highly reactive dopamine and DOPA quinones. Dopamine and DOPA quinones may irreversibly alter protein function through the formation of 5-cysteinyl-catechols on the proteins. Furthermore, the quinone formation is closely linked to other representative hypotheses such as mitochondrial dysfunction, inflammation, oxidative stress, and dysfunction of the ubiquitin-proteasome system, in the pathogenesis of neurodegenerative diseases. Therefore, pathogenic effects of the dopamine quinone have recently focused on dopaminergicneuron-specific oxidative stress. In this article, we primarily review recent studies on the pathogenicity of quinone formation, in addition to several neuroprotective approaches against dopaminequinone-induced dysfunction of dopaminergic neurons.