The Cu 2؉ complexes of the 1-16 and the 1-20 fragments of the Alzheimer's disease-related ␤-amyloid peptide (CuA␤) show significant oxidative activities toward a catechol-like substrate trihydroxylbenzene and plasmid DNA cleavage. The latter reflects possible oxidative stress to biological macromolecules, yielding supporting data to the pathological role of these soluble A␤ fragments. Abnormal metal-ion homeostasis has been closely associated with several neurodegenerative diseases, including Parkinson's, amyotrophic lateral sclerosis, Creutzfeldt-Jakob disease (i.e. human "mad cow" or prion disease), and Alzheimer's disease (AD) 1 (1-4). Because high cytoplasmic concentrations of free metal ions are toxic and potentially lethal, intricate physiological pathways have evolved to transport and distribute metal ions to their targets, which include enzymes and proteins (5). With aging, physiological processes responsible for accurate delivery of metal ions break down and "leakage" of free metal ions can cause toxic effects to cells (6, 7). Divalent ions of redox-active transition metals have often been associated with oxidative stress and closely involved in the chemistry of reactive oxygen species (ROS), including hydrogen peroxide as well as superoxide and hydroxyl radicals (8). Because increase in intracellular concentrations of metal ions is closely related to the effects of aging, oxidative stress, and AD, there is considerable interest in investigating the connection between malfunction of regulatory processes such as metal transport and the presence of ROS with the pathology of AD.The chemistry of redox-active metal complexes of ␤-amyloid peptide (A␤) has been an area of intense focus in the study of AD. The aggregation of A␤ within the neocortex is closely related to the pathology of AD and has been shown to be induced by metal binding (9, 10). The A␤ peptides are generated by the cleavage of the ubiquitous amyloid precursor protein by ␣, ␤, and ␥ secretases (11). A␤ in the form of insoluble plaques contains up to millimolar amounts of Zn 2ϩ , Cu 2ϩ , and Fe 3ϩ in the neocortical region of the brain (8); however, the cause/effect connection of the metallo-A␤ plaques with AD is still under debate (12). The metal coordination environment of the 1-40 and 1-42 peptides has been previously studied and their pH-dependent aggregation reported (10, 13). The results showed that the metal binding seemed to be non-stoichiometric with ϳ3.5 metal ions per pair of aggregated peptides and a cooperative binding pattern as the amount of aggregates increases (8). Because A␤ 1-42 and A␤ have been shown to bind Zn 2ϩ , Fe 3ϩ , and Cu 2ϩ with extremely low apparent dissociation constants by means of quantitative determination of the metal-complex precipitates (8), understanding of the metalbinding domain and its structure may shed light on the chemistry related to the neuropathology of AD.Although the coagulation of the peptide plaques leaves little doubt that interaction with cytoplasmic molecules is unlikely, smaller fragment...

A clear mind: The CuII–β‐amyloid (Aβ) complex has been shown to exhibit enzyme‐like metal‐centered oxidative and hydroxylation catalyses. Metal‐centered oxidation of various neurotransmitters by CuAβ under biomimetic conditions has verified the bio‐relevance of the metal‐centered catalyses and is expected to provide a chemical basis for the better understanding of the etiology of Alzheimer's disease. ROS=reactive oxygen species.

A gray area in gray matter: The CuII complex of a truncated β‐amyloid, CuAβ1–20, catalyzes the oxidation of catechol and the hydroxylation and oxidation of phenol (see picture) with dramatic rate accelerations (≈105–106‐fold increases). The Cu–oxygen chemistry of CuAβ may offer both a better understanding of the chemical effect in the brains of patients who suffer from Alzheimer's disease as well as possible treatment strategies of this disease.

A subfamily of rhodopsin pigments was recently discovered in bacteria and proposed to function as dual-function light-driven H(+)/Na(+) pumps, ejecting sodium ions from cells in the presence of sodium and protons in its absence. This proposal was based primarily on light-induced proton flux measurements in suspensions of Escherichia coli cells expressing the pigments. However, because E. coli cells contain numerous proteins that mediate proton fluxes, indirect effects on proton movements involving endogenous bioenergetics components could not be excluded. Therefore, an in vitro system consisting of the purified pigment in the absence of other proteins was needed to assign the putative Na(+) and H(+) transport definitively. We expressed IAR, an uncharacterized member from Indibacter alkaliphilus in E. coli cell suspensions, and observed similar ion fluxes as reported for KR2 from Dokdonia eikasta. We purified and reconstituted IAR into large unilamellar vesicles (LUVs), and demonstrated the proton flux criteria of light-dependent electrogenic Na(+) pumping activity in vitro, namely, light-induced passive proton flux enhanced by protonophore. The proton flux was out of the LUV lumen, increasing lumenal pH. In contrast, illumination of the LUVs in a Na(+)-free suspension medium caused a decrease of lumenal pH, eliminated by protonophore. These results meet the criteria for electrogenic Na(+) transport and electrogenic H(+) transport, respectively, in the presence and absence of Na(+). The direction of proton fluxes indicated that IAR was inserted inside-out into our sealed LUV system, which we confirmed by site-directed spin-label electron paramagnetic resonance spectroscopy. We further demonstrate that Na(+) transport by IAR requires Na(+) only on the cytoplasmic side of the protein. The in vitro LUV system proves that the dual light-driven H(+)/Na(+) pumping function of IAR is intrinsic to the single rhodopsin protein and enables study of the transport activities without perturbation by bioenergetics ion fluxes encountered in vivo.

A recently discovered rhodopsin ion pump (DeNaR, also known as KR2) in the marine bacterium Dokdonia eikasta uses light to pump protons or sodium ions from the cell depending on the ionic composition of the medium. In cells suspended in a KCl solution, DeNaR functions as a light-driven proton pump, whereas in a NaCl solution, DeNaR conducts light-driven sodium ion pumping, a novel activity within the rhodopsin family. These two distinct functions raise the questions of whether the conformations of the protein differ in the presence of K+ or Na+ and whether the helical movements that result in the canonical E → C conformational change in other microbial rhodopsins are conserved in DeNaR. Visible absorption maxima of DeNaR in its unphotolyzed (dark) state show an 8 nm difference between Na+ and K+ in decyl maltopyranoside micelles, indicating an influence of the cations on the retinylidene photoactive site. In addition, electronic paramagnetic resonance (EPR) spectra of the dark states reveal repositioning of helices F and G when K+ is replaced with Na+. Furthermore, the conformational changes assessed by EPR spin–spin dipolar coupling show that the light-induced transmembrane helix movements are very similar to those found in bacteriorhodopsin but are altered by the presence of Na+, resulting in a new feature, the clockwise rotation of helix F. The results establish the first observation of a cation switch controlling the conformations of a microbial rhodopsin and indicate specific interactions of Na+ with the half-channels of DeNaR to open an appropriate path for ion translocation.