Importance of cardiac cell membranes in vanadate-induced NADH oxidation

In vitro and in silico evaluation of the inhibitory effect of a curcumin-based oxovanadium (IV) complex on alkaline phosphatase activity and bacterial biofilm formation

The scientific interest in the development of novel metal-based compounds as inhibitors of bacterial biofilm-related infections and alkaline phosphatase (ALP) deregulating effects is continuous and rising. In the current study, a novel crystallographically defined heteroleptic V(IV)-curcumin-bipyridine (V-Cur) complex with proven bio-activity was studied as a potential inhibitor of ALP activity and bacterial biofilm. The inhibitory effect of V-Cur was evaluated on bovine ALP, with two different substrates: para-nitrophenyl phosphate (pNPP) and adenosine triphosphate (ATP). The obtained results suggested that V-Cur inhibited the ALP activity in a dose-dependent manner (IC₅₀ = 26.91 ± 1.61 μM for ATP, IC₅₀ = 2.42 ± 0.12 μM for pNPP) exhibiting a mixed/competitive type of inhibition with both substrates tested. The evaluation of the potential V-Cur inhibitory effect on bacterial biofilm formation was performed on Gram (+) bacteria Staphylococcus aureus (S. aureus) and Gram (-) Escherichia coli (E. coli) cultures, and it positively correlated with inhibition of bacterial ALP activity. In silico study proved the binding of V-Cur at eukaryotic and bacterial ALP, and its interaction with crucial amino acids of the active sites, verifying complex's inhibitory potential. The findings suggested a specific anti-biofilm activity of V-Cur, offering a further dimension in the importance of metal complexes, with naturally derived products as biological ligands, as therapeutic agents against bacterial infections and ALP-associated diseases. KEY POINTS: • V-Cur inhibits bovine and bacterial alkaline phosphatases and bacterial biofilm formation. • Alkaline phosphatase activity correlates with biofilm formation. • In silico studies prove binding of the complex on alkaline phosphatase.

Vanadium redox cycling, lipid peroxidation and co-oxygenation of benzo(a)pyrene-7,8-dihydrodiol

Mechanism of lipid peroxidation triggered by vanadium in human term placental microsomes was reinvestigated in vitro. Production of lipid peroxyl radicals was estimated from co-oxygenation of benzo(a)pyrene and benzo(a)pyrene-7,8-dihydrodiol. Vanadyl(IV), but not vanadate(V) caused a dose-dependent co-oxygenation. Vanadate(V) required the presence of reduced nicotinamide adenine dinucleotide phosphate to trigger co-oxygenation of benzo(a)pyrene-7,8-dihydrodiol. To determine the role of pre-formed lipid hydroperoxides, the results obtained with partially peroxidized linoleic acid were compared with those of fresh linoleate. Superoxide dismutase inhibited the co-oxygenation of reaction when fresh linoleic acid was used. To further characterize the role of superoxide anion-radical in the vanadium redox cycling, the increase of optical density of vanadate(V) dissolved in Tris buffer was measured at 328 nm during the addition of KO2. The rate of this reaction producing peroxy-vanadyl complex was decreased by superoxide dismutase, especially, in the presence of catalase. It is suggested that vanadium catalyzes two separate processes, both leading to enhanced lipid peroxidation: (i) initiation, dependent on superoxide and triggered by peroxy-vanadyl; (ii) propagation, dependent on pre-formed lipid hydroperoxide not sensitive to superoxide dismutase. It is postulated that the vanadium-triggered initiation of lipid peroxidation may be crucial for toxicity in organs with limited endogenous lipid peroxidation.

Flavoenzymes reduce vanadium(V) and molecular oxygen and generate hydroxyl radical

ESR spectroscopic evidence is presented for the formation of vanadium(IV) in the reduction of vanadium(V) by three typical, NADPH-dependent, flavoenzymes: glutathione reductase, lipoyl dehydrogenase, and ferredoxin-NADP+ oxidoreductase. The vanadium(V)-reduction mechanism appears to be an enzymatic one-electron reduction process. Addition of superoxide dismutase (SOD) showed that the generation of vanadium(IV) does not involve the superoxide (O2-) radical significantly. Measurements under anaerobic atmosphere showed, however, that the enzymes-vanadium-NADPH mixture can cause the reduction of molecular oxygen to generate H2O2. The H2O2 and vanadium(IV) thus formed react to generate hydroxyl (.OH) radical. The .OH formation is inhibited strongly by catalase and to a lesser degree by SOD, but it is enhanced by exogenous H2O2, suggesting the occurrence of a Fenton-like reaction. The inhibition of vanadium(IV) formation by N-ethylmaleimide indicates that the SH group on the flavoenzyme's cystine residue plays an important role in the enzyme's vanadium(V) reductase function. These results thus reveal a new property of the above-mentioned, NADPH-dependent flavoenzymes--their function as vanadium(V) reductases, as well as that as generators of .OH radical in the vanadium(V) reduction mechanism.

Glutathione reductase functions as vanadate(V) reductase

The oxidation of NADPH by vanadate(V) in the presence of glutathione reductase showed typical enzymatic kinetics. The oxidation was inhibited by N-ethylmaleimide, a glutathione reductase inhibitor. Superoxide dismutase had no significant effect on the oxidation, indicating noninvolvement of the superoxide radical. The vanadate(V) reduction was found to be a one-electron transfer process. These results suggest a new pathway for vanadate(V) metabolism and a new function of glutathione reductase.

Hydroxyl radicals is not a significant intermediate in the vanadate-stimulated oxidation of NAD(P)H by O2

The vanadate-stimulated oxidation of NADH by an enzymatic flux of O2- is inhibited by superoxide dismutase, but not by catalase. Keller et al. (1989, Free Radical Biol. Med. 6, 15-22) observed inhibition by catalase presumably because they used a commercial preparation contaminated with superoxide dismutase. Their proposal, that H2O2 and hydroxyl radical play significant roles in vanadate-stimulation of NAD(P)H oxidation, may be discounted on the basis of these and of previously reported results.

Vanadate-stimulated oxidation of NAD(P)H

Vanadate stimulates the oxidation of NAD(P)H by biological membranes because such membranes contain NAD(P)H oxidases which are capable of reducing dioxygen to O2- and because vanadate catalyzes the oxidation of NAD(P)H by O2-, by a free radical chain mechanism. Dihydropyridines, such as reduced nicotinamide mononucleotide (NMNH), which are not substrates for membrane-associated NAD(P)H oxidases, are not oxidized by membranes plus vanadate unless NAD(P)H is present to serve as a source of O2-. When [NMNH] greatly exceeds [NAD(P)H], in such reaction mixtures, one can observe the oxidation of many molecules of NMNH per NAD(P)H consumed. This reflects the chain length of the free radical chain mechanism. We have discussed the mechanism and significance of this process and have tried to clarify the pertinent but confusing literature.