Periodic NADH oxidase activity associated with an endoplasmic reticulum fraction from pig liver. Response to micromolar concentrations of retinol

Multiple proteins with single activities or a single protein with multiple activities: the conundrum of cell surface NADH oxidoreductases

Reduction of the cell-impermeable tetrazolium salt WST-1 has been used to characterise two plasma membrane NADH oxidoreductase activities in human cells. The trans activity, measured with WST-1 and the intermediate electron acceptor mPMS, utilises reducing equivalents from intracellular sources, while the surface activity, measured with WST-1 and extracellular NADH, is independent of intracellular metabolism. Whether these two activities involve distinct proteins or are inherent to a single protein is unclear. In this work, we have attempted to address this question by examining the relationship between the trans and surface WST-1-reducing activities and a third well-characterised family of cell surface oxidases, the ECTO-NOX proteins. Using blue native-polyacrylamide gel electrophoresis, we have identified a complex in the plasma membranes of human 143B osteosarcoma cells responsible for the NADH-dependent reduction of WST-1. The dye-reducing activity of the 300 kDa complex was attributed to a 70 kDa NADH oxidoreductase activity that cross-reacted with antisera against the ECTO-NOX protein CNOX. Differences in enzyme activities and inhibitor profiles between the WST-1-reducing NADH oxidoreductase enzyme in the presence of NADH or mPMS and the ECTO-NOX family are reconciled in terms of the different purification methods and assay systems used to study these proteins.

Spectroscopic Analyses of Oscillations in ECTO-NOX-Catalyzed Oxidation of NADH

Spectroscopic strategies that substantiate periodic oscillations in low rates of NADH oxidation exhibited by ECTO-NOX proteins at the animal and plant cell surface are described. Both continuous display and discontinuous rate determinations exhibit the oscillations but continuous displays lack sufficient resolution to discern details. A procedure is documented where rates are determined by least squares analyses of traces recorded over 1 min at intervals of 1.5 min. These traces recapitulate the continuous displays but offer an opportunity to reliably estimate changes in reaction rates over short time intervals not afforded by the continuous traces. Results from previously used rate determination over 5 min intervals are included for comparison. Turbidity is identified as the major contributor to losses in resolution. Even highly purified NOX preparations tend to aggregate to form turbid suspensions. With turbid suspensions, double beam or dual wavelength instrumentation where the sample is placed immediately adjacent to the photomultiplier tube are required to reduce losses in resolution from turbidity. Also required are high levels of synchronous ECTO-NOX function. Blue or red (plants) light, small molecules (i.e., melatonin) and autosynchrony alone or in combination were used to synchronize the oscillations. Special problems posed by preparations containing more than one ECTO-NOX form and where the different ECTO-NOX forms do not cross entrain are discussed.

Specificity of coenzyme Q inhibition of an aging-related cell surface NADH oxidase (ECTO-NOX) that generates superoxide

Our laboratories have described a novel class of ectoproteins at the cell surface with both NADH or hydroquinone oxidase (NOX) and protein disulfide-thiol interchange activities (ECTO-NOX proteins). The two activities exhibited by these proteins alternate to generate characteristic patterns of oscillations where the period length is independent of temperature. The period length for the constitutive ECTO-NOX is 24 min. Here we describe a distinctive age-related ECTO-NOX (arNOX) whose activity is blocked by coenzyme Q10. arNOX occurs exclusively in aged cells and tissues. The period length of the oscillations is 26 min. Rather than reducing 1/2 O2 to H2O, electrons are transferred to O2 to form superoxide. Superoxide formation was demonstrated by superoxide dismutase-sensitive reduction of ferricytochrome c and by reduction of a superoxide-specific tetrazolium salt. Quinone inhibition was given by coenzymes Q8, 9 and Q10 but not by Q0, Q2, Q4, Q6 or 7. The arNOX provides a mechanism to propagate reactive oxygen species generated at the cell surface to surrounding cells and circulating lipoproteins of importance to atherogenesis. Inhibition of arNOX by dietary coenzyme Q10 provides a rational basis for dietary coenzyme 10 use to retard aging-related arterial lesions.

A site-directed mutagenesis analysis of tNOX functional domains

Constitutive NADH oxidase proteins of the mammalian cell surface exhibit two different activities, oxidation of hydroquinones (or NADH) and protein disulfide-thiol interchange which alternate to yield oscillatory patterns with period lengths of 24 min. A drug-responsive tNOX (tumor-associated NADH oxidase) has a period length of about 22 min. The tNOX cDNA has been cloned and expressed. These two proteins are representative of cycling oxidase proteins of the plant and animal cell surface. In this report, we describe a series of eight amino acid replacements in tNOX which, when expressed in Escherichia coli, were analyzed for enzymatic activity, drug response and period length. Replacement sites selected include six cysteines that lie within the processed plasma membrane (34 kDa) form of the protein, and amino acids located in putative drug and adenine nucleotide (NADH) binding domains. The latter, plus two of the cysteine replacements, resulted in a loss of enzymatic activity. The recombinant tNOX with the modified drug binding site retained activity but the activity was no longer drug-responsive. The four remaining cysteine replacements were of interest in that both activity and drug response were retained but the period length for both NADH oxidation and protein disulfide-thiol interchange was increased from 22 min to 36 or 42 min. The findings confirm the correctness of the drug and adenine nucleotide binding motifs within the tNOX protein and imply a potential critical role of cysteine residues in determining the period length.