Investigation Of Redox-active Metal Complexes And Their Neuropathalogical Relevance

Neurodegeneration is termed as the progressive loss of structures or functions of neurons, which eventually results in death of neurons. The main neurodegenerative diseases include Parkinson's disease and Alzheimer's disease. Prion disease is another neurodegenerative form with infectivity. Although many neurodegenerative diseases are manifested by the aggregation of the amyloidogenic proteins (e.g.,?-amyloid or A?in Alzheimer's disease (AD), alpha-synuclein (?-syn) in Parkinson's disease (PD), and prion protein in Prion disease), oxidative stress has also been implicated in the pathogenesis of neurodegenerative diseases. Consequently, metal-induced oxidative stree processes have been an area under active pursuit. The implication of metal-induced oxidative stress is extremely broad, ranging from acceleration of the formation of reactive oxygen species (ROS) in the presence of redox-active metals such as copper and iron, mitochondria function impairment, and neuronal membrane damage through lipid peroxidation to depletion of vital intracellular species. Based on the above considerations, we have carried out the following studies:1. A forefront of the research on Alzheimer's disease is the interaction of amyloid beta (A?) peptides with redox-active metal ions and the biological relevance of the A?-metal complexes to neuronal cell loss and homeostasis of essential metals and other cellular species. This work is concerned with the kinetic and mechanistic studies of the ascorbic acid (AA) oxidation by molecular oxygen, which is facilitated by Cu(?) complexes with A?(1-16), A?(1-42), and aggregates of A?(1-42). The mechanism for the AA oxidation in which the oxidation states of the copper center in the A?complexes alternate between +2 and +1 is proposed. The catalytic activity of Cu(?) complex towards AA oxidation was found to decrease in the order of free Cu(?)>A?(1-16)-Cu(?)> A?(1-42)-Cu(?)> Cu(?) complexed by the A?oligomer/fibril mixture> Cu(?) in A?fibrils. Unlike free Cu(?), in the presence of AA, A?-Cu(?) complexes facilitate the reduction of oxygen by producing H2O2 as a final product. Although Cu(?) bound to oligomeric and fibrous A?aggregates is less effective than free Cu(?) and the monomeric A?-Cu(?) complex in producing ROS, in vivo the Cu(?)-containing Apoligomers and fibrils might be more biologically relevant given their strong association with cell membranes and the closer proximity of ROS to cell membranes.2. Parkinson's disease, one of the leading neurodegenerative diseases, is characterized by the degeneration of dopaminergic neurons in the substantia nigra (SN), iron overload in SN and the presence of Lewy bodies of abnormal cytoplasmic inclusions that are mainly composed of alpha-synuclein (?-syn) in the dying neurons.?-syn is known to bind Cu(?). The redox potential of the?-syn-Cu(?) complex was determined to be 0.018 V (vs. Ag/AgCl). Furthermore, the Cu(?) center can be readily reduced to Cu(?), and possible reactions of?-syn-Cu(?) with cellular species (e.g., O2, ascorbic acid) have been investigated. In addition, the generated H2O2 was demonstrated to reduce the viability of the neuroblastoma SY-HY5Y cells. Our results thus suggest that oxidative stress is at least partially responsible for the loss of dopaminergic cells in PD brain and reveal the multifaceted role of the?-syn-Cu(?) complex in oxidative stress associated with PD symptoms.3. Iron has been proven, in vitro, to catalyze the oxidation of DA by oxygen to generate neuromelanin. In this work, we investigated the formation of ternary complexes of DA-Fe(?) with ATP. The results reveal that the ligation by ATP almost totally shut down the catalytic oxidation of DA via blockage of the oxygen access to the Fe(?) center. In addition, ATP could decrease greatly the cell toxicity of DA or DA-Fe(?) complex. In particular, ATP is the chemical energy for cell function and is abundant in cells (1?10 mM). It is possible that ATP-DA-Fe(?) exists in vivo. In relevance to PD is the significant depletion of ATP in neuronal cell of PD afflicted brain. Therefore, the depletion of ATP might be a crucial cause for PD.4. A misfolded form of the prion protein (PrP) leads to the development of the prion diseases. The linkage between PrP and bioavailable Cu(?) has been well established. PrP contains four highly conserved repeats of PHGGGWGQ octapeptide (OP) sequence within its N-terminal domain for Cu(?) sequestration. In vitro studies have shown that when the concentration ratio between PrP and Cu(?) is close to 1:1, three or four histidines in the OP4 domain coordinate the single Cu(?) center (OP4-Cu(?)). When the Cu(?) concentration is four or more equivalents of Cu(?), each of the four OPs binds one Cu(?), giving rise to OP4-Cu(?)4. We report here that the redox activity (cycling) of Cu(?) is highly dependent on the mode in which Cu(?) is coordinated within the PrP octarepeat domain. It is indicated that PrP possesses the unique ability to quench the copper redox activity in the form of OP4-Cu(?) but promote the controlled H2O2 production in the form of OP4-Cu(?)4. The results demonstrated herein are helpful for understanding the function of PrP in vivo.5. MTs paly an important role in regulation of essential metals, detoxification of heavy metals, and scavenging of free radicals. Recently, MTs have captured the attention of many researchists. In this study, modulation of metal release from MTs by the glutathione redox couple has been investigated. Upon separation of Zn2+ generated from the reaction mixture of MTs and glutathione redox couple (GSH/GSSG) with a centrifugal filter membrane, electrochemical technique was used in tandom with UV-vis spectrophotometry to characterize Zn2+ content and the concomitant conversion between free thiol groups and disulfide bonds. The new approach is demonstrated to be well suited for investigation of redox regulation of MT (e. g. MT/a-syn-Cu(?) and MT/A?-Cu(?)).