NADH fluorescence lifetime analysis of the effect of magnesium ions on ALDH2

Mitochondrial matrix pH as a decisive factor in neurometabolic imaging

Alterations of cellular bioenergetics are a common feature in most neurodegenerative disorders. However, there is a selective vulnerability of different brain regions, cell types, and even mitochondrial populations to these metabolic disturbances. Thus, the aim of our study was to establish and validate an in vivo metabolic imaging technique to screen for mitochondrial function on the subcellular level. Based on nicotinamide adenine dinucleotide (phosphate) fluorescence lifetime imaging microscopy [NAD(P)H FLIM], we performed a quantitative correlation to high-resolution respirometry. Thereby, we revealed mitochondrial matrix pH as a decisive factor in imaging NAD(P)H redox state. By combining both parameters, we illustrate a quantitative, high-resolution assessment of mitochondrial function in metabolically modified cells as well as in an amyloid precursor protein-overexpressing model of Alzheimer's disease. Our metabolic imaging technique provides the basis for dissecting mitochondrial deficits not only in a range of neurodegenerative diseases, shedding light onto bioenergetic failures of cells remaining in their metabolic microenvironment.

Structural Insights into the Drosophila melanogaster Retinol Dehydrogenase, a Member of the Short-Chain Dehydrogenase/Reductase Family

The 11-cis-retinylidene chromophore of visual pigments isomerizes upon interaction with a photon, initiating a downstream cascade of signaling events that ultimately lead to visual perception. 11-cis-Retinylidene is regenerated through enzymatic transformations collectively called the visual cycle. The first and rate-limiting enzymatic reaction within this cycle, i.e., the reduction of all-trans-retinal to all-trans-retinol, is catalyzed by retinol dehydrogenases. Here, we determined the structure of Drosophila melanogaster photoreceptor retinol dehydrogenase (PDH) isoform C that belongs to the short-chain dehydrogenase/reductase (SDR) family. This is the first reported structure of a SDR that possesses this biologically important activity. Two crystal structures of the same enzyme grown under different conditions revealed a novel conformational change of the NAD⁺ cofactor, likely representing a change during catalysis. Amide hydrogen-deuterium exchange of PDH demonstrated changes in the structure of the enzyme upon dinucleotide binding. In D. melanogaster, loss of PDH activity leads to photoreceptor degeneration that can be partially rescued by transgenic expression of human RDH12. Based on the structure of PDH, we analyzed mutations causing Leber congenital amaurosis 13 in a homology model of human RDH12 to obtain insights into the molecular basis of RDH12 disease-causing mutations.

Fluorescence lifetime analysis and effect of magnesium ions on binding of NADH to human aldehyde dehydrogenase 1

Aldehyde dehydrogenase 1 (ALDH1A1) catalyzes the oxidation of toxic aldehydes to carboxylic acids. Physiologic levels of Mg(2+) ions decrease ALDH1 activity in part by increasing NADH binding affinity to the enzyme. By using time-resolved fluorescence spectroscopy, we have resolved the fluorescent lifetimes (τ) of free NADH in solution (τ=0.4 ns) and two enzyme-bound NADH states (τ=2.0 ns and τ=7.7 ns). We used this technique to investigate the effects of Mg(2+) ions on the ALDH1A1-NADH binding characteristics and enzyme catalysis. From the resolved free and bound NADH fluorescence signatures, the KD values for both NADH conformations in ALDH1A1 ranged from about 24 μM to 1 μM for Mg(2+) ion concentrations of 0-6000 μM, respectively. The rate constants for dissociation of the enzyme-NADH complex ranged from 0.03 s(-1) (6000 μM Mg(2+)) to 0.30s(-1) (0 μM Mg(2+)) as determined by addition of excess NAD(+) to prevent re-association of NADH and resolving the real-time NADH fluorescence signal. During the initial reaction of enzyme with NAD(+) and butyraldehyde, there was an immediate rise in the NADH fluorescence, due to the formation of bound NADH complexes, with a constant steady-state rate of production of free NADH. As the Mg(2+) ion concentration was increased, there was a consistent decrease of the enzyme catalytic turnover from 0.31 s(-1) (0 μM Mg(2+)) to 0.050 s(-1) (6000 μM Mg(2+)) and a distinct shift in steady-state conformational population from one that favors the ALDH1-NADH complex with the shorter fluorescence lifetime (33% excess) in the absence of magnesium ion to one that favors the ALDH1-NADH complex with the longer fluorescence lifetime (13% excess) at 6000 μM Mg(2+). This shift in conformational population at higher Mg(2+) ion concentrations and to lower enzyme activity may be due to longer residence time of the NADH in the ALDH1 pocket. The results from monitoring enzyme catalysis in the absence of magnesium suggests that the ALDH1-NADH complex with the shorter fluorescence lifetime is the form initially produced, and the complex with the longer fluorescence lifetime is produced through isomerization.