Inactivation of aldehyde dehydrogenase in intact rat liver mitochondria by dopamine

Structural understanding of T cell receptor triggering

The T cell receptor (TCR) is one of the most complicated receptors in mammalian cells, and its triggering mechanism remains mysterious. As an octamer complex, TCR comprises an antigen-binding subunit (TCRαβ) and three CD3 signaling subunits (CD3ζζ, CD3δε, and CD3γε). Engagement of TCRαβ with an antigen peptide presented on the MHC leads to tyrosine phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) in CD3 cytoplasmic domains (CDs), thus translating extracellular binding kinetics to intracellular signaling events. Whether conformational change plays an important role in the transmembrane signal transduction of TCR is under debate. Attracted by the complexity and functional importance of TCR, many groups have been studying TCR structure and triggering for decades using diverse biochemical and biophysical tools. Here, we synthesize these structural studies and discuss the relevance of the conformational change model in TCR triggering.

TCR-pMHC bond conformation controls TCR ligand discrimination

A major unanswered question is how a TCR discriminates between foreign and self-peptides presented on the APC surface. Here, we used in situ fluorescence resonance energy transfer (FRET) to measure the distances of single TCR–pMHC bonds and the conformations of individual TCR–CD3ζ receptors at the membranes of live primary T cells. We found that a TCR discriminates between closely related peptides by forming single TCR–pMHC bonds with different conformations, and the most potent pMHC forms the shortest bond. The bond conformation is an intrinsic property that is independent of the binding affinity and kinetics, TCR microcluster formation, and CD4 binding. The bond conformation dictates the degree of CD3ζ dissociation from the inner leaflet of the plasma membrane via a positive calcium signaling feedback loop to precisely control the accessibility of CD3ζ ITAMs for phosphorylation. Our data revealed the mechanism by which a TCR deciphers the structural differences among peptides via the TCR–pMHC bond conformation.

Neurotoxic effects of tetrahydroisoquinolines and underlying mechanisms

Tetrahydropapaveroline (THP), a neurotoxic tetrahydroisoquinoline alkaloid formed by condensation between dopamine and dopaldehyde, has been speculated to cause Parkinson's disease and also to contribute to alcohol dependence. Having two catechol moieties, THP may readily undergo oxidation to form an o-quinone intermediate with concomitant production of reactive oxygen species, which can cause neuronal cell death and DNA damage. This review will deal with the current knowledge of neurotoxic effects of this endogenous alkaloid and underlying biochemical mechanisms.

Proteomic identification of dopamine-conjugated proteins from isolated rat brain mitochondria and SH-SY5Y cells

Dopamine oxidation has been previously demonstrated to cause dysfunction in mitochondrial respiration and membrane permeability, possibly related to covalent modification of critical proteins by the reactive dopamine quinone. However, specific mitochondrial protein targets have not been identified. In this study, we utilized proteomic techniques to identify proteins directly conjugated with (14)C-dopamine from isolated rat brain mitochondria exposed to radiolabeled dopamine quinone (150 microM) and differentiated SH-SY5Y cells treated with (14)C-dopamine (150 microM). We observed a subset of rat brain mitochondrial proteins that were covalently modified by (14)C-dopamine, including chaperonin, ubiquinol-cytochrome c reductase core protein 1, glucose regulated protein 75/mitochondrial HSP70/mortalin, mitofilin, and mitochondrial creatine kinase. We also found the Parkinson's disease associated proteins ubiquitin carboxy-terminal hydrolase L1 and DJ-1 to be covalently modified by dopamine in both brain mitochondrial preparations and SH-SY5Y cells. The susceptibility of the identified proteins to covalent modification by dopamine may carry implications for their role in the vulnerability of dopaminergic neurons in Parkinson's disease pathogenesis.

Study on the ability of 1,2,3,4-tetrahydropapaveroline to cause oxidative stress: Mechanisms and potential implications in relation to parkinson's disease

Tetrahydropapaveroline (THP) is a compound derived from dopamine monoamine oxidase-mediated metabolism, particularly present in the brain of parkinsonian patients receiving L-dopa therapy, and is capable of causing dopaminergic neurodegeneration. The aim of this work was to evaluate the potential of THP to cause oxidative stress on mitochondrial preparations and to gain insight into the molecular mechanisms responsible for its neurotoxicity. Our data show that THP autoxidation occurs with a continuous generation of hydroxyl radicals (*OH) and without the involvement of the Fenton reaction. The presence of ascorbate enhances this process by establishing a redox cycle, which regenerates THP from its quinolic forms. It has been shown that the production of *OH is not affected by the presence of either ferrous or ferric iron. Although THP does not affect lipid peroxidation, it is capable of reducing the high levels of thiobarbituric acid-reactive substances obtained in the presence of ascorbate and/or iron. However, THP autoxidation in the presence of ascorbate causes both an increase in protein carbonyl content and a reduction in protein-free thiol content. THP also increases protein carbonyl content when the autoxidation occurs in the presence of iron. The remarkable role played by ascorbate in the production of oxidative stress by THP autoxidation is of particular interest.

Purification and characterization of aldehyde dehydrogenase from rat liver mitochondrial matrix

Aldehyde dehydrogenase (EC 1.2.1.3) has been purified to homogeneity from Sprague-Dawley rat liver mitochondrial matrix; its specific activity with propionaldehyde (1 mM at pH 9.0) is 1.4 mumol/min/mg. It has a native molecular weight of ca. 260,000 daltons, a subunit weight of 54,000 daltons and separates into two bands on isoelectric focusing (pI, 5.15 and 5.30); its extinction coefficient at 280 nm for 1 mg/ml solution is 1.2 and 280/260 nm ratio is 1.6. The enzyme prefers NAD over NADP as the coenzyme; the Km for NADP (67,000 microM) is three orders of magnitude greater than that for NAD (61 microM); the Km for acetaldehyde is 2 microM and for propionaldehyde is 0.8 microM at pH 7.0. The enzyme is reversibly inhibited by chloral (Ki = 3 microM) but is resistant to disulfiram inhibition. In addition another aldehyde dehydrogenase, first observed in the mitochondrial matrix during isoelectric focusing, has been partially purified. It has a Km for propionaldehyde of 0.7 mM and an isoelectric point of 6.4. Its activity with glutamic-gamma-semialdehyde (Km = 0.13 mM) is ca. 20 times higher than with propionaldehyde identifying the enzyme as glutamic-gamma-semialdehyde dehydrogenase (EC 1.5.1.12).