Scavenging of Cation Radicals of the Visual Cycle Retinoids by Lutein, Zeaxanthin, Taurine, and Melanin.

In the retina, retinoids involved in vision are under constant threat of oxidation, and their oxidation products exhibit deleterious properties. Using pulse radiolysis, this study determined that the bimolecular rate constants of scavenging cation radicals of retinoids by taurine are smaller than 2 × 107 M-1s-1 whereas lutein scavenges cation radicals of all three retinoids with the bimolecular rate constants approach the diffusion-controlled limits, while zeaxanthin is only 1.4-1.6-fold less effective. Despite that lutein exhibits greater scavenging rate constants of retinoid cation radicals than other antioxidants, the greater concentrations of ascorbate in the retina suggest that ascorbate may be the main protectant of all visual cycle retinoids from oxidative degradation, while α-tocopherol may play a substantial role in the protection of retinaldehyde but is relatively inefficient in the protection of retinol or retinyl palmitate. While the protection of retinoids by lutein and zeaxanthin appears inefficient in the retinal periphery, it can be quite substantial in the macula. Although the determined rate constants of scavenging the cation radicals of retinol and retinaldehyde by dopa-melanin are relatively small, the high concentration of melanin in the RPE melanosomes suggests they can be scavenged if they are in proximity to melanin-containing pigment granules.

Scavenging of Retinoid Cation Radicals by Urate, Trolox, and α-, ß-, γ-, and δ-Tocopherols.

Retinoids are present in human tissues exposed to light and under increased risk of oxidative stress, such as the retina and skin. Retinoid cation radicals can be formed as a result of the interaction between retinoids and other radicals or photoexcitation with light. It has been shown that such semi-oxidized retinoids can oxidize certain amino acids and proteins, and that α-tocopherol can scavenge the cation radicals of retinol and retinoic acid. The aim of this study was to determine (i) whether ß-, γ-, and δ-tocopherols can also scavenge these radicals, and (ii) whether tocopherols can scavenge the cation radicals of another form of vitamin A-retinal. The retinoid cation radicals were generated by the pulse radiolysis of benzene or aqueous solution in the presence of a selected retinoid under oxidizing conditions, and the kinetics of retinoid cation radical decays were measured in the absence and presence of different tocopherols, Trolox or urate. The bimolecular rate constants are the highest for the scavenging of cation radicals of retinal, (7 to 8) × 109 M-1·s-1, followed by retinoic acid, (0.03 to 5.6) × 109 M-1·s-1, and retinol, (0.08 to 1.6) × 108 M-1·s-1. Delta-tocopherol is the least effective scavenger of semi-oxidized retinol and retinoic acid. The hydrophilic analogue of α-tocopherol, Trolox, is substantially less efficient at scavenging retinoid cation radicals than α-tocopherol and urate, but it is more efficient at scavenging the cation radicals of retinoic acid and retinol than δ-tocopherol. The scavenging rate constants indicate that tocopherols can effectively compete with amino acids and proteins for retinoid cation radicals, thereby protecting these important biomolecules from oxidation. Our results provide another mechanism by which tocopherols can diminish the oxidative damage to the skin and retina and thereby protect from skin photosensitivity and the development and/or progression of changes in blinding retinal diseases such as Stargardt's disease and age-related macular degeneration (AMD).

Free radical coupling of o-semiquinones uncovered.

As a rule, o-semiquinones decay through disproportionation leading to equimolar amounts of catechol and o-quinone products. However, the o-semiquinone 1S generated by pulse radiolysis oxidation of the eumelanin precursor 5,6-dihydroxyindole (1) decays with second-order kinetics to generate broad visible chromophores that are incompatible with the predicted absorption of 5,6-indolequinone (1Q). Using an integrated chemical, pulse radiolytic and computational approach as well as deuterium labeling, we show herein that 1S and related 5,6-dihydroxyindole semiquinones decay mainly by a free radical coupling mechanism. This conclusion was supported by the inverse kinetic isotope effect observed with deuterated 1S, the identification of unprecedented dihydrobiindole products by one-electron oxidation of 1, the good matching of simulated absorption profiles of free radical coupling products of 1S with experimental spectra, and a detailed computational analysis of the kinetics and thermodynamics of the disproportionation equilibrium and free radical coupling of 1S versus 1-1Q coupling. These results disclose, to the best of our knowledge, the first example of free radical dimerization of o-semiquinones outcompeting the classic disproportionation-driven catechol-quinone coupling and suggest that this hitherto unrecognized process may be of broader relevance than previously believed.

Potency of inhibition of human DNA topoisomerase I by flavones assessed through physicochemical parameters.

DNA topoisomerases, enzymes involved in DNA replication and transcription, are known as targets for anticancer drugs. Among the various types of topoisomerase inhibitors, flavones (F) have been identified as promising compounds. In this study, it is shown that the potency of flavones acting as topoisomerase I inhibitors can be ranked according to their redox properties and their 3D structure. Linear correlations were observed between the topoisomerase I inhibition activity exerted by five flavones (chrysin, apigenin, kaempferol, fisetin, quercetin) and experimental and theoretical redox parameters of F. Moreover, theoretical calculations of the dihedral angle O(1)-2-1'-2' in the flavone molecules indicate the importance of their structural and steric features in their potency as topoisomerase I inhibitors. It is suggested that the flavones might interact with the DNA-topoisomerase I complex after their oxidation into quinones via autoxidation, enzymatic oxidation, or reactions with reactive oxygen species. Our investigation opens a new strategy quantitatively based on redox and 3D structural parameters in the search for the most active flavones as anticancer drug candidates inhibiting topoisomerase I.

Cyclic structural motifs in 5,6-dihydroxyindole polymerization uncovered: biomimetic modular buildup of a unique five-membered macrocycle.

An unprecedented 5,6-dihydroxyindole macrocycle (4) featuring a rigid twisted backbone was obtained by biomimetic oxidative cross-coupling of the 2,2'-biindole 2 and triindole 3. A putative reaction intermediate, 2-quinone, was detected and characterized by pulse radiolysis and DFT calculations. Discovery of 4 indirectly supports for the first time theoretically predicted cyclic structural motifs as potential eumelanin building blocks.

Lack of visible chromophore development in the pulse radiolysis oxidation of 5,6-dihydroxyindole-2-carboxylic acid oligomers: DFT investigation and implications for eumelanin absorption properties.

The structural factors underlying the peculiar optical properties and visible chromophore of eumelanin biopolymers are largely uncharted. It is known that synthetic eumelanins from 5,6-dihydroxyindole are black and display a featureless UV-visible absorption spectrum, whereas those from 5,6-dihydroxyindole-2-carboxylic acid (1) are lighter in color and exhibit a distinct band around 310 nm, but the origin of this difference has never been addressed in detail. Recently, we showed that 5,6-dihydroxyindole dimers generate on pulse radiolysis oxidation strongly absorbing transients with intense maxima in the 500-600 nm region, which have been attributed to planar extended quinone methide species. We now report the unexpectedly different behavior of three oligomers from 1, namely, the 4,4'-biindolyl 2, the 4,7'-biindolyl 3, and the 4,7':4',7''-terindolyl 4. Pulse radiolysis oxidation of 2-4 led initially to semiquinone intermediates exhibiting similar absorption maxima at 360-380 nm. Semiquinone absorption decay followed second-order kinetics (2k = 1.4 x 10(8), 3.2 x 10(8), and 1.4 x 10(8) M(-1) s(-1) for 2, 3, and 4, respectively) but did not lead to significant chromophore development in the visible region. Similar absorption traces were obtained from monomer 1. DFT calculations predicted 5,6-dihydroxyindolyl-5,6-indolequinone structures with significant dihedral twists across the interunit single bonds for the most stable two-electron oxidation products of 2 and 3. The computed absorption spectra consistently featured strong bands around 310 nm but little or no absorption in the visible region. It is suggested that the effective conjugation length in oligomeric/polymeric eumelanin components from 1 may be controlled by hindered rotation around inter-ring bonds preventing planarization of the continuous array of indole units. This may provide an explanation for the difference in the absorption properties of polymers from the two key eumelanin monomers.

Dopamine quinone chemistry: a study of the influence of amide, amidine and guanidine substituents [-NH-CX-Y] on the mode of reaction.

The influence of N-substituents on the mode of reaction of ortho-quinones generated by oxidation of N-substituted dopamine derivatives has been studied. Ortho-quinones with amide, urea or guanidine side chains are relatively stable, with evidence of rearrangement to para-quinomethanes. The N-methylthiourea derivative rapidly cyclises giving a bicyclic product . The trichloromethylamidine derivative also rapidly cyclises but in this case gives a spirocyclic derivative . In contrast to the transient formation of spirocyclic products by other ortho-quinones derived from dopamine derivatives, e.g., , the product is stable and has been isolated and fully characterised.

Evidence consistent with the requirement of cresolase activity for suicide inactivation of tyrosinase.

Tyrosinase is a mono-oxygenase with a dinuclear copper catalytic center which is able to catalyze both the ortho-hydroxylation of monophenols (cresolase activity) and the oxidation of catechols (catecholase activity) yielding ortho-quinone products. Tyrosinases appear to have arisen early in evolution and are widespread in living organisms where they are involved in several processes, including antibiosis, adhesion of molluscs, the hardening of the exoskeleton of insects, and pigmentation. Tyrosinase is the principal enzyme of melanin formation in vertebrates and is of clinical interest because of the possible utilization of its activity for targeted treatment of malignant melanoma. Tyrosinase is characterised by an irreversible inactivation that occurs during the oxidation of catechols. In a recent publication we proposed a mechanism to account for this feature based on the ortho-hydroxylation of catecholic substrates, during which process Cu(II) is reduced to Cu(0) which no longer binds to the enzyme and is eliminated (reductive elimination). Since this process is dependent on cresolase activity of tyrosinase, a strong prediction of the proposed inactivation mechanism is that it will not be exhibited by enzymes lacking cresolase activity. We show that the catechol oxidase readily extracted from bananas (Musa cavendishii) is devoid of cresolase activity and that the kinetics of catechol oxidation do not exhibit inactivation. We also show that a species with the molecular mass of the putative cresolase oxidation product is formed during tyrosinase oxidation of 4-methylcatechol. The results presented are entirely consistent with our proposed mechanism to account for suicide-inactivation of tyrosinase.

The mechanism of suicide-inactivation of tyrosinase: a substrate structure investigation.

Tyrosinase is a copper-containing mono-oxygenase, widely distributed in nature, able to catalyze the oxidation of both phenols and catechols to the corresponding ortho-quinones. Tyrosinase is characterised by a hitherto unexplained irreversible inactivation which occurs during the oxidation of catechols. Although the corresponding catechols are formed during tyrosinase oxidation of monophenols, inactivation in the presence of monophenolic substrates is minimal. Previous studies have established the kinetic features of the inactivation reaction which is first-order in respect of the enzyme concentration. The inactivation reaction exhibits the same pH-profile and saturation properties as the oxidation reaction, classing the process as a mechanism-based suicide inactivation. The recent elucidation of the crystallographic structure of tyrosinase has stimulated a new approach to this long-standing enigma. Here we report the results of an investigation of the tyrosinase-catalysed oxidation of a range of hydroxybenzenes which establish the structural requirements associated with inactivation. We present evidence for an inactivation mechanism based on catechol hydroxylation, with loss of one of the copper atoms at the active site. The inactivation mechanism involves two linked processes occurring in situ: (a) catechol presentation resulting in alpha-oxidation, and (b) deprotonation of an adjacent group. On the basis of our experimental data we believe that a similar mechanism may account for the inhibitory action of resorcinols.

Chemical, pulse radiolysis and density functional studies of a new, labile 5,6-indolequinone and its semiquinone.

The chemical and spectroscopic characterization of 5,6-indolequinones and their semiquinones, key transient intermediates in the oxidative conversion of 5,6-dihydroxyindoles to eumelanin biopolymers, is a most challenging task. In the present paper, we report the characterization of a novel, relatively long-lived 5,6-indolequinone along with its semiquinone using an integrated chemical, pulse radiolytic, and computational approach. The quinone was obtained by oxidation of 5,6-dihydroxy-3-iodoindole (1a) with o-chloranil in cold ethyl acetate or aqueous buffer: it displayed electronic absorption bands around 400 and 600 nm, was reduced to 1a with Na2S2O4, and reacted with o-phenylenediamine to give small amounts of 3-iodo-1H-pyrrolo[2,3-b]phenazine (2). The semiquinone exhibited absorption maxima at 380 nm (sh) and 520 nm and was detected as the initial species produced by pulse radiolytic oxidation of 1a at pH 7.0. DFT investigations indicated the 6-phenoxyl radical and the N-protonated radical anion as the most stable tautomers for the neutral and anion forms of the semiquinone, respectively. Calculated absorption spectra in water gave bands at 350 (sh) and 500 nm for the neutral form and at 310 and 360 (sh) nm for the anion. Disproportionation of the semiquinone with fast second-order kinetics (2k = 1.1 x 1010 M-1 s-1) gave a chromophore with absorption bands resembling those of chemically generated 1a quinone. Computational analysis predicted 1a quinone to exist in vacuo as the quinone-methide tautomer, displaying low energy transitions at 380 and 710 nm, and in water as the o-quinone, with calculated absorption bands around 400 and 820 nm. A strong participation of a p orbital on the iodine atom in the 360-380 nm electronic transitions of the o-quinone and quinone-methide was highlighted. The satisfactory agreement between computational and experimental electronic absorption data would suggest partitioning of 1a quinone between the o-quinone and quinone-methide tautomers depending on the medium.

Studies of carotenoid one-electron reduction radicals.

The relative reduction potentials of a variety of carotenoids have been established by monitoring the reaction of carotenoid radical anion (CAR1(*-)) with another carotenoid (CAR2) in hexane and benzene. This order is consistent with the reactivities of the carotenoid radical anions with porphyrins and oxygen in hexane. In addition, investigation of the reactions of carotenoids with reducing radicals in aqueous 2% Triton-X 100, such as carbon dioxide radical anion (CO2(*-)), acetone ketyl radical (AC(*-)) and the corresponding neutral radical (ACH(*)), reveals that the reduction potentials for beta-carotene and zeaxanthin lie in the range -1950 to -2100 mV and those for astaxanthin, canthaxanthin and beta-apo-8'-carotenal are more positive than -1450 mV. This illustrates that the presence of a carbonyl group causes the reducing ability to decrease. The radical cations have been previously shown to be strong oxidising agents and we now show that the radical anions are very strong reducing agents.

Short-lived quinonoid species from 5,6-dihydroxyindole dimers en route to eumelanin polymers: integrated chemical, pulse radiolytic, and quantum mechanical investigation.

The transient species formed by oxidation of three dimers of 5,6-dihydroxyindole (1), a major building block of the natural biopolymer eumelanin, have been investigated. Pulse radiolytic oxidation of 5,5',6,6'-tetrahydroxy-2,4'-biindolyl (3) and 5,5',6,6'-tetrahydroxy-2,7'-biindolyl (4) led to semiquinones absorbing around 450 nm, which decayed with second-order kinetics (2k=2.8x10(9) and 1.4x10(9) M-1 s-1, respectively) to give the corresponding quinones (500-550 nm). 5,5',6, 6'-Tetrahydroxy-2,2'-biindolyl (2), on the other hand, furnished a semiquinone (lamdamax=480 nm) which disproportionated at a comparable rate (2k=3x10(9) M-1 s-1) to give a relatively stable quinone (lamdamax=570 nm). A quantum mechanical investigation of o-quinone, quinonimine, and quinone methide structures of 2-4 suggested that oxidized 2-4 exist mainly as 2-substituted extended quinone methide tautomers. Finally, an oxidation product of 3 was isolated for the first time and was formulated as the hydroxylated derivative 5 arising conceivably by the addition of water to the quinone methide intermediate predicted by theoretical analysis. Overall, these results suggest that the oxidation chemistry of biindolyls 2-4 differs significantly from that of the parent 1, whereby caution must be exercised before concepts that apply strictly to the mode of coupling of 1 are extended to higher oligomers.

Carotenoid radical anions and their protonated derivatives.

In this study, we report the protonation reactions for astaxanthin and canthaxanthin radical anions in methanol, alkaline methanol, and aqueous 2% Triton X-100 at different pH values. The pKa values for the corresponding alpha-hydroxy radical derivatives of astaxanthin, canthaxanthin, and beta-apo-8'-carotenal were estimated in 2% Triton X-100. Also, the effects of the microenvironment and the structure of the carotenoids on the protonation rate constant are discussed.

Mechanistic studies of melanogenesis: the influence of N-substitution on dopamine quinone cyclization.

The influence of side-chain structure on the mode of reaction of ortho-quinone amines has been investigated with a view, ultimately, to developing potential methods of therapeutic intervention by manipulating the early stages of melanogenesis. Four N-substituted dopamine derivatives have been prepared and quinone formation studied using pulse radiolysis and tyrosinase-oximetry. Ortho-quinones with an amide or urea side chain were relatively stable, although evidence for slow formation of isomeric para-quinomethanes was observed. A thiourea derivative cyclized fairly rapidly (k = 1.7/s) to a product containing a seven-membered ring, whereas a related amidine gave more rapidly (k approximately 2.5 x 10(2)/s) a stable spirocyclic product. The results suggest that cyclization of amides, ureas and carbamates (NHCO-X; X = R, NHR or OR) does not occur and is not, therefore, a viable approach to the formation of tyrosinase-activated antimelanoma prodrugs. It is also concluded that for N-acetyldopamine spontaneous ortho-quinone to para-quinomethane isomerization is slow.

Pulse radiolysis study of the interaction of retinoids with peroxyl radicals.

Vitamin A (retinol) and its derivatives-retinal and retinoic acid-are known for their ability to inhibit lipid peroxidation. Antioxidant actions of retinoids have been attributed to chain-breaking by scavenging of peroxyl radicals. Based on chemical analysis of retinoic acid degradation products formed during microsomal lipid peroxidation, it was previously suggested that retinoids interact with peroxyl radicals forming free carbon-centered radical adducts. However, it can be argued that such a mode of antioxidant action of retinoids is not sufficient to fully explain their effectiveness at inhibiting lipid peroxidation, which in many systems is comparable to, or even exceeds, that of alpha-tocopherol. In order to elucidate the mechanism of interaction of retinoids with peroxyl radicals, (trichloromethyl)peroxyl radical was generated by pulse radiolysis, and its interactions with retinoids solubilized in Triton X-100 micelles were followed by kinetic absorption spectroscopy. All retinoids--retinol, retinal, and retinoic acid--interacted with the peroxyl radical, and at least two transient products were detected. One of these products, absorbing at 590 nm, was identified as retinoid cation radical. Therefore, we postulate that, apart from formation of radical adducts, retinoids may also scavenge peroxyl radicals by electron transfer.

Oxidation of N-substituted dopamine derivatives: irreversible formation of a spirocyclic product.

Oxidation of amide, urea and guanidinium derivatives of dopamine gives relatively stable ortho-quinones whereas oxidation of corresponding thioamide and amidinium derivatives rapidly and quantitatively gives novel bicyclic and spirocyclic products formed via the corresponding ortho-quinone.

Spectroscopic properties and reactivity of free radical forms of A2E.

A pyridinium bisretinoid (A2E) is the only identified blue-absorbing chromophore of retinal lipofuscin that has been linked to its aerobic photoreactivity and phototoxicity. Pulse radiolysis has been used to study both the one-electron oxidation and the one-electron reduction of A2E in aqueous micellar solutions. The reduction to the semireduced A2E (lambda(max) broad and between 500 and 540 nm) was achieved with formate radicals and the subsequent decay of A2E* was slow (over hundreds of milliseconds) via complex kinetics. The long lifetime of the A2E* should facilitate its reactions with other biomolecules. For example, with oxygen, the A2E* produced the superoxide radical anion with a rate constant of 3 x 10(8) M(-1) s(-1). The A2E was also reduced by the NAD radical, the corresponding rate constant being 2.3 x 10(8) M(-1) s(-1). Other experiments showed that the one-electron reduction potential of A2E lies in the range -640 to -940 mV. The semioxidized form of A2E (lambda(max) 590 nm) was formed via oxidation with the Br2*- radical and had a much shorter lifetime than the semireduced form. With strongly oxidizing peroxyl radicals (CCl3O2*) our kinetic data suggest the formation of a radical adduct followed by dissociation to the semioxidized A2E. With milder oxidizing peroxyl radicals such as that from methanol, our results were inconclusive. In benzene we observed an efficient oxidation of zeaxanthin to its radical cation by the A2E radical cation; this may be relevant to a detrimental effect of A2E in vision.

Are dietary carotenoids beneficial? Reactions of carotenoids with oxy-radicals and singlet oxygen.

Carotenoids play diverse roles in biology and medicine. Both the quenching of singlet oxygen (energy transfer) and interaction with oxy-radicals (electron transfer, H-atom transfer and addition reactions) are key processes in understanding many of these roles. Much previous work in 'simple' solvents is reviewed and new results in cell membrane models are presented. The possible consequences of using carotenoids as dietary supplements are discussed.