Inhibition of the mechanical activity of mouse ileum by cactus pear (Opuntia Ficus Indica, L, Mill.) fruit extract and its pigment indicaxanthin

We investigated, using an organ bath technique, the effects of a hydrophilic extract from Opuntia ficus indica fruit pulp (cactus fruit extract, CFE) on the motility of mouse ileum, and researched the extract component(s) responsible for the observed responses. CFE (10-320 mg of fresh fruit pulp equivalents/mL of organ bath) reduced dose-dependently the spontaneous contractions. This effect was unaffected by tetrodotoxin, a neuronal blocker, N(omega)-nitro-l-arginine methyl ester, a nitric oxide synthase blocker, tetraethylammonium, a potassium channel blocker, or atropine, a muscarinic receptor antagonist. CFE also reduced the contractions evoked by carbachol, without affecting the contractions evoked by high extracellular potassium. Indicaxanthin, but not ascorbic acid, assayed at concentrations comparable with their content in CFE, mimicked the CFE effects. The data show that CFE is able to exert direct antispasmodic effects on the intestinal motility. The CFE inhibitory effects do not involve potassium channels or voltage-dependent calcium channels but rather pathways of calcium intracellular release. The fruit pigment indicaxanthin appears to be the main component responsible for the CFE-induced effects.

Kinetics of the lipoperoxyl radical-scavenging activity of indicaxanthin in solution and unilamellar liposomes

The reaction of the phytochemical indicaxanthin with lipoperoxyl radicals generated in methyl linoleate methanol solution by 2,2'-azobis(2,4-dimethylvaleronitrile), and in aqueous soybean phosphatidylcholine unilamellar liposomes by 2,2'-azobis(2-amidinopropane)hydrochloride, was studied. The molecule acts as a chain-terminating lipoperoxyl radical scavenger in solution, with a calculated inhibition constant of 3.63 x 10(5) M(-1) s(-1), and a stoichiometric factor approaching 2. Indicaxanthin incorporated in liposomes prevented lipid oxidation, inducing clear-cut lag periods and decrease of the propagation rate. Both effects were concentration-dependent, but not linearly related to the phytochemical concentration. The consumption of indicaxanthin during liposome oxidation was remarkably delayed, the lower the concentration the longer the time-interval during which it remained in its native state. Indicaxanthin and alpha-tocopherol, simultaneously incorporated in liposomes, exhibited cooperative antioxidant effects and reciprocal protective interactions. The extent of synergism decreased at the increase of the ratio (indicaxanthin)/(alpha-tocopherol). A potential antioxidant mechanism of indicaxanthin is discussed in the context of the chemistry of the molecule, and of the possible reactivity of a short-lived intermediate.

Cytoprotective effects of the antioxidant phytochemical indicaxanthin in β-thalassemia red blood cells

Antioxidant phytochemicals are investigated as novel treatments for supportive therapy in beta-thalassemia. The dietary indicaxanthin was assessed for its protective effects on human beta-thalassemic RBCs submitted in vitro to oxidative haemolysis by cumene hydroperoxide. Indicaxanthin at 1.0-10 microM enhanced the resistance to haemolysis dose-dependently. In addition, it prevented lipid and haemoglobin (Hb) oxidation, and retarded vitamin E and GSH depletion. After ex vivo spiking of blood from thalassemia patients with indicaxanthin, the phytochemical was recovered in the soluble cell compartment of the RBCs. A spectrophotometric study showed that indicaxanthin can reduce perferryl-Hb generated in solution from met-Hb and hydrogen peroxide (H2O2), more effectively than either Trolox or vitamin C. Collectively our results demonstrate that indicaxanthin can be incorporated into the redox machinery of beta-thalassemic RBC and defend the cell from oxidation, possibly interfering with perferryl-Hb, a reactive intermediate in the hydroperoxide-dependent Hb degradation. Opportunities of therapeutic interest for beta-thalassemia may be considered.

Bioactive components of caper (Capparis spinosa L.) from Sicily and antioxidant effects in a red meat simulated gastric digestion

An increasing body of evidence on the association between adherence to the Mediterranean diet and healthy status is being accumulated. Floral buds of Capparis spinosa L. are commonly used in the Mediterranean cuisine as flavoring for meat and other foods. The present study evaluated bioactive components and antioxidant activity of Sicilian capers stabilized in salt. Whereas alpha-tocopherol was absent, low levels of gamma-tocopherol and vitamin C were measured. With reference to one serving size (8.6 g of capers), rutin was 13.76 mg, isothiocyanates, recently acknowledged as anticarcinogen phytochemicals, were 42.14 micromol, total phenols were 4.19 mg of gallic acid equivalents (GAE), and the total antioxidant potential measured using the [2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)] diammonium salt (ABTS) cation radical decolorization assay was 25.8 micromol of Trolox equivalents. The antioxidative activity of a caper hydrophilic extract was assessed in a number of assays. The extract at 3.5 and 7.0 microM GAE exhibited a dose-dependent peroxyl radical scavenging activity in a methyl linoleate methanol solution oxidized by azo initiator, and reduced hypervalent iron myoglobin species formed from met-Mb an H 2O 2, at 180 microM GAE. The hydrophilic extract, at 70-280 microM GAE, caused a dose-dependent inhibition of lipid autoxidation in heated red meat, incubated with simulated gastric fluid for 180 min. In the same model rutin tested at a concentration corresponding to its content in the extract was ineffective, and alpha-tocopherol at 25 microM was poorly effective. The hydrophilic extract (70 microM GAE) prevented the consumption of the co-incubated alpha-tocopherol, whereas lipid oxidation was inhibited for the experimental time, suggesting cooperative interactions between extract components and the vitamin. The findings encourage the use of caper with foods that contribute oxidizable lipids in view of the association between dietary oxidized lipids and risk of oxidative stress-based diseases.

Biothiols, taurine, and lipid-soluble antioxidants in the edible pulp of Sicilian cactus pear (Opuntia ficus-indica) fruits and changes of bioactive juice components upon industrial processing

Biothiols, taurine, and flavonols, as well as tocopherols and carotenoids have been assessed in the edible pulp of Sicilian red (Sanguigna), yellow (Surfarina), and white (Muscaredda) cultivars of cactus pear. The yellow cultivar has the highest level of reduced glutathione (GSH, 8.1 +/- 0.78 mg/100 g pulp), whereas the white cultivar showed the highest amount of cysteine (1.21 +/- 0.12 mg/100 g pulp). Taurine accounted for 11.7 +/- 1.0 mg/100 g in the yellow pulp, while lower levels were measured in the others. With the exception of kaempferol in the yellow cultivar (2.7 +/- 0.2 microg/100 g pulp), the edible pulp of cactus pear was not a source of flavonols. Very low amounts of lipid-soluble antioxidant vitamins such as vitamin E and carotenoids were measured in all cultivars. As a consequence of industrial processing, a total loss of GSH and beta-carotene and a net decrease of vitamin C and cysteine were revealed in the fruit juice, whereas betalains, taurine, and vitamin E appeared to be less susceptible to degradation.

Antioxidant betalains from cactus pear (Opuntia ficus-indica) inhibit endothelial ICAM-1 expression

It has been suggested that some pigments would have antioxidant properties and that their presence in dietary constituents would contribute to reduce the risk of oxidative stress-correlated diseases. Among others, inflammatory response depends on redox status and may implicate oxidative stress. Vascular endothelial cells are a direct target of oxidative stress in inflammation. We have tested the impact of the free radical scavenger and antioxidant properties of betalains from the prickle pear in an in vitro model of endothelial cells. Here we show the capacity of betalains to protect endothelium from cytokine-induced redox state alteration, through ICAM-1 inhibition.

Increased resistance to oxidation of betalain-enriched human low density lipoproteins

Betalains are natural pigments recently considered as compounds with potential antioxidative properties. In this work, ex vivo plasma spiking of pure either betanin or indicaxanthin, followed by isolation of low density lipoprotein (LDL), and measurement of its resistance to copper-induced oxidation, has been used to research if these betalains can bind to LDL and prevent oxidation of LDL lipids. When pooled human plasma from 10 healthy volunteers was incubated in the presence of 25-100 microM either betanin or indicaxanthin, incorporation of both compounds in LDL was observed, with a maximum binding of 0.52 +/- 0.08, and 0.51 +/- 0.06 nmoles of indicaxanthin and betanin, respectively, per mg LDL protein. Indicaxanthin-enriched and betanin-enriched LDL were more resistant than homologous native LDL to copper-induced oxidation, as assessed by the elongation of the induction period. The incorporated indicaxanthin, however, appeared twice as effective as betanin in increasing the length of the lag phase, while both compounds did not affect the propagation rate. Both betalains were consumed during the inhibition period of lipid oxidation, and delayed consumption of LDL-beta carotene. Indicaxanthin, but not betanin, prevented vitamin E consumption at the beginning of LDL oxidation, and prolonged the time of its utilization. The resistance of LDL to oxidation when vitamin E and indicaxanthin acted separately in a sequence, was lower than that measured when they were allowed to act in combination, indicating some synergistic interaction between the two molecules. No prooxidant effect over a large concentration range of either betanin or indicaxanthin was observed, when either betalain was added to the LDL system undergoing a copper-induced oxidation. These results show than indicaxanthin and betanin may bind to LDL, and are highly effective in preventing copper-induced lipid oxidation. Interaction with vitamin E appears to add a remarkable potential to indicaxanthin in the protection of LDL. Although molecular mechanisms remain uncompletely understood, various aspects of the action of betanin and indicaxanthin in preventing LDL lipid oxidation are discussed.

The chemistry of melatonin's interaction with reactive species

Melatonin has been shown to be an effective antioxidant in a number of experimental models both in vitro and in vivo. Considering the data available, it is now clear that the indoleamine is involved in antioxidative mechanisms more complex than originally envisaged. These range from the direct radical scavenging of a variety of radicals and reactive species to the control and/or modulation of a number of processes which may trigger a redox imbalance between antioxidant and prooxidant species. This review focuses on the direct radical scavenging activity of melatonin and provides a summary of the mechanisms of the reactions between the indoleamine and reactive species in pure chemical solutions. These actions likely account for at least some of the protective actions of melatonin under conditions of high oxidative stress.

Protective effect of melatonin against cytotoxic actions of malondialdehyde: an in vitro study on human erythrocytes

Malondialdehyde (MDA), a by-product of the oxidation of polyunsaturated fatty acids, is strongly cytotoxic. Here we report the in vitro ability of melatonin to protect intact human erythrocytes against the damage induced by the exposure to MDA. MDA at 20 microM caused marked variations in the red blood cell (RBC) membrane. High molecular weight fluorescent adducts were formed within minutes with membrane proteins. A 6-hr incubation led to the oxidation of membrane lipids, as reflected by the formation of conjugated diene (CD) lipid hydroperoxides and oxidation of vitamin E, and to an increase of the high molecular weight fluorescent adducts, which were an indication of MDA finally generated in the cells. Functional damage to the membrane was evident as a leakage of K+ ions into the incubation medium, and an increased resistance to osmotic lysis. A time-dependent hemolysis was observed by exposure of RBCs to 20 microM MDA for 6-12 hr. Melatonin was not a substrate for MDA, therefore it was not able to prevent the early formation of the adducts from the reaction of the MDA in the medium with membrane proteins. Melatonin, however, concentration-dependent prevented the formation of CD lipid hydroperoxides. As a consequence of counteracting the membrane lipid oxidation, the indoleamine prevented the loss of vitamin E and the increase of the fluorescent proteinaceous adducts observed after a 6-hr exposure to MDA. Melatonin also inhibited the K+ loss and returned to normal the osmotic resistance of the erythrocyte in the osmotic fragility test. By protecting membrane lipids and proteins, melatonin effectively prevented the MDA-induced time-dependent hemolysis. In the light of the known radical scavenging properties of melatonin, mechanisms of the cytoprotective effects of melatonin in our system are discussed.

Oxidation of melatonin by oxoferryl hemoglobin: a mechanistic study

Reaction of melatonin with the hypervalent iron centre of oxoferryl hemoglobin, produced in aqueous solution from methemoglobin and H2O2, has been investigated at 37 degrees C and pH 7.4, by absorption spectroscopy. The reaction results in reduction of the oxoferryl moiety with formation of a heme-ferric containing hemoprotein. Stopped-flow spectrophotometric measurements provide evidence that the reduction of oxoferryl-Hb by melatonin is first-order in oxoferryl-Hb and first-order in melatonin. The bimolecular reaction constant at pH 7.4 and 37 degrees C is 112 +/- 1.0 M(-1) s(-1). Two major oxidation products from melatonin have been found by gas chromatography-mass spectroscopy: the cyclic compound 1,2,3,3a,8,8a-hexahydro-1-acetyl-5-methoxy-3a-hydroxypyrrolo[2,3-b]indole (cyclic 3-hydroxy-melatonin), and N-acetyl-N'-formyl 5-methoxykynuramine (AFMK). The percentage yield of the two major products appears dependent on the ratio [oxoferryl-Hb]:[melatonin]--the higher the ratio the higher the yield of AFMK. The observed stoichiometry oxoferryl-Hb(reduced):melatonin(consumed) is 2, when the ratio [oxoferryl-Hb]:[melatonin] is 1:1, but appears >2 at higher molar ratios. The reduction of the hypervalent iron of the oxoferryl moiety may be consistent with an oxidation of melatonin by two one-electron steps.

Reaction of melatonin with hemoglobin-derived oxoferryl radicals and inhibition of the hydroperoxide-induced hemoglobin denaturation in red blood cells

Melatonin has been shown to act as a radical scavenger in various chemical and biological model systems in vitro. Kinetic evidence is now provided showing that melatonin inhibits the irreversible degradation of hemoglobin (Hb), when incubated with red blood cells exposed to the oxidant activity of cumene hydroperoxide (cumOOH). A decrease of heme loss and accumulation of soluble methemoglobin (met-Hb) are explained in terms of the interaction of the indoleamine with perferryl Hb (Hb[Fe(IV)=O]), a highly reactive Hb-derived radical species responsible for the irreversible Hb degradation. A kinetic study, in pure chemical solution, showed that melatonin can effectively reduce the oxoferryl heme group of perferryl-Hb, thus forming met-Hb. The reducing activity of melatonin is of the same order as that of Trolox, the water-soluble vitamin E analog. This novel radical-scavenging activity of melatonin may contribute to the previously observed protective effects of melatonin in ischemia-reperfusion injury.

Oral supplements of vitamin E improve measures of oxidative stress in plasma and reduce oxidative damage to LDL and erythrocytes in beta-thalassemia intermedia patients

Fifteen beta-thalassemia intermedia patients, not requiring chronic transfusional therapy, were monitored in order to check their antioxidant status, and the lipid oxidation products in plasma, LDL, and erythrocytes before and during a 9-month oral treatment with 600 mg/day vitamin E. The low level of vitamin E, and high level of malondialdehyde in plasma clearly tended to normalize after three months (P < .001), and were quite similar to control after six months. The abnormally low level of vitamin E in LDL and the four times higher than control basal level of conjugated dienes (LDL-CD), were not modified after three months of treatment. Significant changes of LDL-VE (P < .05) and of the basal LDL-CD (P < .001) were evident after six months. LDL-VE was within the normal range after nine months, whereas LDL-CD still appeared twice as higher than control. Plasma vitamin A, ascorbate, beta-carotene, and lycopene increased markedly at the end of the trial (P < .005). The level of vitamin E in red blood cells was normalized after six months of supplementation. A decrease of the baseline value of conjugated dienes was observed after nine months, although it remained 1.4-fold higher than control. The RBC count and hematocrit appeared higher at the end of the trial (P < .05 and P < .001, respectively). The hemoglobin value did not show variations. A shift to normal of the resistance of erythrocytes to osmotic lysis was observed. Our findings provide evidence that an oral treatment with vitamin E improves the antioxidant/oxidant balance in plasma, LDL particles, and red blood cells, and counteracts lipid peroxidation processes in beta-thalassemia intermedia patients.

Mechanism of reaction of melatonin with human myeloperoxidase

Recently, it was suggested that melatonin (N-acetyl-5-methoxytryptamine) is oxidized by activated neutrophils in a reaction most probably involving myeloperoxidase (Biochem. Biophys. Res. Commun. (2000) 279, 657-662). Myeloperoxidase (MPO) is the most abundant protein of neutrophils and is involved in killing invading pathogens. To clarify if melatonin is a substrate of MPO, we investigated the oxidation of melatonin by its redox intermediates compounds I and II using transient-state spectral and kinetic measurements at 25 degrees C. Spectral and kinetic analysis revealed that both compound I and compound II oxidize melatonin via one-electron processes. The second-order rate constant measured for compound I reduction at pH 7 and pH 5 are (6.1 +/- 0.2) x 10(6) M(-1) s(-1) and (1.0 +/- 0.08) x 10(7) M(-1) s(-1), respectively. The rates for the one-electron reduction of compound II back to the ferric enzyme are (9.6 +/- 0.3) x 10(2) M(-1) s(-1) (pH 7) and (2.2 +/- 0.1) x 10(3) M(-1) s(-1) (pH 5). Thus, melatonin is a much better electron donor for compound I than for compound II. Steady-state experiments showed that the rate of oxidation of melatonin is dependent on the H(2)O(2) concentration, is not affected by superoxide dismutase, and is quickly terminated by sodium cyanide. Melatonin can markedly inhibit the chlorinating activity of MPO at both pH 7 and pH 5. The implication of these findings in the activated neutrophil is discussed.

Oxidative stress after moderate to extensive burning in humans

Lipid peroxidation products, lipid antioxidants, and hematologic and blood chemistry changes were evaluated in plasma of patients after acute burning injury involving 10% (n=8), 20% (n=8), and 40% (n=5) of total body surface area (TBSA), 24 h after burning (baseline) up to 30 days after. Markedly increased plasma levels of malondialdehyde (MDA) were observed at baseline in all patients, according to the extent of the injury, then the values declined progressively. However, levels of MDA remained above normal up to 30 days even in less injured patients. On the other hand, the plasma level of conjugated diene lipid hydroperoxides was only slightly higher than control at the baseline, then dropped under the control value in all patients. Cholesterol showed a marked fall at baseline, followed by a rapidly progressive decrease, indicating a massive loss of circulating lipids by the acute thermal injury. Because of such an extensive and rapidly spreading oxidative degradation of lipids, decomposition of conjugated diene hydroperoxides, produced in early stages of the peroxidation process, occurs, so these compounds cannot be a suitable index to value lipid oxidation in burned patients. Aldehydic products of lipid peroxidation act as endotoxins, causing damage to various tissues and organs. Damage to liver and decrease of erythrocyte survival were assessed by increased plasma levels of asparate and alanine transaminases, within 7-15 days after injury, and by a decreased number of red blood cells, which remained under the normal value at 30 days. A marked decrease of lipid antioxidants, beta-carotene, vitamin A and vitamin E was observed at baseline. The level of beta-carotene remained low in all patients at the end of the 30-day observation. A complete recovery of vitamin A did not occur at 30 days post-burn, even in the patients with 10% of burned TBSA. Plasma levels of vitamin E decreased significantly in 1-7 days after burn in all patients, but these levels increased thereafter, with almost total recovery at 30 days. These data show evidence of a marked, long-lasting oxidant/antioxidant imbalance in burned patients, in accordance with the severity of the injury, which is also reflected as systemic oxidant stress.

Melatonin protects human red blood cells from oxidative hemolysis: new insights into the radical-scavenging activity

Antioxidant activity of melatonin in human erythrocytes, exposed to oxidative stress by cumene hydroperoxide (cumOOH), was investigated. CumOOH at 300 microM progressively oxidized a 1% suspension of red blood cells (RBCs), leading to 100% hemolysis in 180 min. Malondialdehyde and protein carbonyls in the membrane showed a progressive increase, as a result of the oxidative damage to membrane lipids and proteins, reaching peak values after 30 and 40 min, respectively. The membrane antioxidant vitamin E and the cytosolic reduced glutathione (GSH) were totally depleted in 20 min. As a consequence of the irreversible oxidative damage to hemoglobin (Hb), hemin accumulated into the RBC membrane during 40 min. Sodium dodecyl sulfate (SDS) gel electrophoresis of membrane proteins showed a progressive loss of the cytoskeleton proteins and formation of low molecular weight bands and protein aggregates, with an increment of the intensity of the Hb band. Melatonin at 50 microM strongly enhanced the RBC resistance to oxidative lysis, leading to a 100% hemolysis in 330 min. Melatonin had no effect on the membrane lipid peroxidation, nor prevented the consumption of glutathione (GSH) or vitamin E. However, it completely inhibited the formation of membrane protein carbonyls for 20 min and hemin precipitation for 10 min. The electrophoretic pattern provided further evidence that melatonin delayed modifications to the membrane proteins and to Hb. In addition, RBCs incubated for 15 min with 300 microM cumOOH in the presence of 50 microM melatonin were less susceptible, when submitted to osmotic lysis, than cells incubated in its absence. Extraction and high-performance liquid chromatography (HPLC) analysis showed a much more rapid consumption of melatonin during the first 10 min of incubation, then melatonin slowly decreased up to 30 min and remained stable thereafter. Equilibrium partition experiments showed that 15% of the melatonin in the incubation mixture was recovered in the RBC cytosol, and no melatonin was extracted from RBC membrane. However, 35% of the added melatonin was consumed during RBC oxidation. Hydroxyl radical trapping agents, such as dimethylsulfoxide or mannitol, added into the assay in a 1,000 times molar excess, did not vary melatonin consumption, suggesting that hydroxyl radicals were not involved in the indole consumption. Our results indicate that melatonin is actively taken up into erythrocytes under oxidative stress, and is consumed in the defence of the cell, delaying Hb denaturation and release of hemin. RBCs are highly exposed to oxygen and can be a site for radical formation, under pathological conditions, which results in their destruction. A protective role of melatonin should be explored in hemolytic diseases.

Interactions of melatonin with membrane models: portioning of melatonin in AOT and lecithin reversed micelles

The interaction of melatonin with water containing either sodium bis (2-ethylhexyl) sulfosuccinate (AOT) or soybean phosphatidylcholine (lecithin) reversed micelles has been investigated by UV absorption spectroscopy, at a molar ratio of melatonin:surfactant 1:800 for AOT and 1:400 for lecithin reversed micelles, and by varying the water:surfactant molar ratio (R). Our results suggest that in the presence of domains from apolar organic solvent to surfactant and to water, melatonin positions itself in the micellar phase, with a preferential location in the surfactant polar head group domain, independent of the nature of the surfactant and the amount of water encapsulated into the micellar core. Effects are due to the hydrophilic and lipophilic moieties of melatonin. The effectiveness of melatonin as an electron donor and free radical scavenger has been recently recognized. While supporting the hypothesis that melatonin may provide antioxidant protection without the benefit of receptors, present findings may suggest that the molecule could easily scavenge aqueous as well as lipophilic radicals.

Oxidative modification of low-density lipoprotein and atherogenetic risk in beta-thalassemia

We investigated the oxidative state of low-density lipoprotein (LDL) in patients with beta-thalassemia to determine whether there was an association with atherogenesis. Conjugated diene lipid hydroperoxides (CD) and the level of major lipid antioxidants in LDL, as well as modified LDL protein, were evaluated in 35 beta-thalassemia intermedia patients, aged 10 to 60, and compared with age-matched healthy controls. Vitamin E and beta-carotene levels in LDL from patients were 45% and 24% of that observed in healthy controls, respectively. In contrast, the mean amount of LDL-CD was threefold higher and lysil residues of apo B-100 were decreased by 17%. LDL-CD in thalassemia patients showed a strong inverse correlation with LDL vitamin E (r = -0.784; P <.0001), while a negative trend was observed with LDL-beta-carotene (r = -0.443; P =.149). In the plasma of thalassemia patients, malondialdehyde (MDA), a byproduct of lipid peroxidation, was increased by about twofold, while vitamin E showed a 52% decrease versus healthy controls. LDL-CD were inversely correlated with plasma vitamin E (r = -0.659; P <.0001) and correlated positively with plasma MDA (r = 0.621; P <. 0001). Plasma ferritin was positively correlated with LDL-CD (r = 0.583; P =.0002). No correlation was found between the age of the patients and plasma MDA or LDL-CD. The LDL from thalassemia patients was cytotoxic to cultured human fibroblasts and cytotoxicity increased with the content of lipid peroxidation products. Clinical evidence of mild to severe vascular complications in nine of the patients was then matched with levels of LDL-CD, which were 36% to 118% higher than the mean levels of the patients. Our results could account for the incidence of atherogenic vascular diseases often reported in beta-thalassemia patients. We suggest that the level of plasma MDA in beta-thalassemia patients may represent a sensitive index of the oxidative status of LDL in vivo and of its potential atherogenicity.

Oxidation resistance of LDL is correlated with vitamin E status in beta-thalassemia intermedia

The alteration of the oxidant/antioxidant balance may affect the susceptibility of low density lipoproteins (LDL) to oxidation in haemolytic disorders such as thalassemia. Thirty patients affected by beta-thalassemia intermedia were examined, and compared with age-matched healthy controls. The mean amount of vitamin E in the thalassemic LDL was lower than control (p < 0.0001), either when it was calculated on the base of LDL protein (61% decrease) or cholesterol (25% decrease). The LDL resistance to Cu2+-induced oxidation, evaluated as the length of the lag phase before the onset of conjugated diene (CD) lipid hydroperoxide production, was 20% lower than control. Other parameters of LDL susceptibility to oxidation, such as the rate of lipid peroxidation, Rp, and the total amount of conjugated dienes produced, CDmax, were only slightly lower than control, which can be explained by a lower content of peroxidable lipids in the thalassemic LDL. Total LDL cholesterol was 1.08 x 10(3) and 2.07 x 10(3) mol/mol LDL in thalassemic and in control LDL, respectively. The length of the lag phase in thalassemic LDL shows a strongly positive correlation with its vitamin E content (r = 0.732; p < 0.0001). The r2-value of 0.53 provides evidence that more than 50% of the lag phase is determined by vitamin E. Oxidizability of LDL lipids may explain 22-24% of the lag phase, as calculated by the inverse correlation between the length of the lag phase and CDmax (r = -0.474; p = 0.008; r2 = 0.22) and Rp (r = -0.499; p = 0.005; r2 = 0.24). In multiple regression analysis, the lag phase was predictable to 66% by vitamin E plus CDmax, and to 60% by vitamin E plus Rp. Plasma vitamin E was 53% lower in thalassemia patients compared to control and positively correlated with vitamin E in the LDL (r = 0.677; p < 0.0001). None of the correlations above were observed in control subjects. In conclusion, beta-thalassemia is associated with very low levels of vitamin E in plasma and in LDL, a condition that renders these particles more susceptible to in vitro oxidative modification and may account for atherogenesis-related vascular diseases described in thalassemia. The present data on a statistically significant correlation between abnormally low vitamin E and oxidizability of LDL contribute substantially to the hypothesis that vitamin E is a pathophysiologically important determinant of antioxidative protection of LDL.

Antioxidant reactions of all-trans retinol in phospholipid bilayers: effect of oxygen partial pressure, radical fluxes, and retinol concentration

Lipoperoxyl radical-scavenging activity of retinol in unilamellar soybean phosphatidylcholine liposomes was studied under a variety of conditions to appreciate to what extend retinol may be considered an effective antioxidant. Peroxidation, initiated by 2 mM 2,2'-azobis(amidino-propane)hydrochloride (AAPH), was carried out at 160 torr O2 or at 15 torr O2, in the absence or in the presence of 10 to 40 mM retinol. As evaluated by the length of the inhibition periods, t(inh), and by the ratio between the inhibition and propagation rate, R(inh)/R(p), the antioxidant activity of retinol was higher at 15 torr O2 than at 160 torr O2. The consumption rate of retinol was markedly faster at 160 torr O2 than at 15 torr O2 and increased with the increase of retinol concentration under both oxygen tensions. When liposome peroxidation was carried out under N2, retinol consumption was independent of retinol concentration. Peroxyl radicals oxidize retinol to 5,6-retinol epoxide. The ratio between 5,6-epoxide formed and the retinol consumed was markedly higher at 15 torr O2 than under air and decreased with the increased retinol concentrations. When butylated hydroxytoluene was included into the liposomal suspension, most of the consumed retinol was converted into 5,6-epoxide. Liposomes were incubated at 15 torr O2, in the presence of 0.5 to 10 mM AAPH. The antioxidant effectiveness of 40 mM retinol, as measured by the R(inh)/R(p) ratio, increased with the increase of the radical fluxes. The results suggest, besides radical trapping, that a major consumption of retinol during lipid oxidation occurs through self-oxidation reactions, which are concentration- and oxygen-dependent. A decreased self-oxidation makes retinol a better lipoperoxyl radical scavenger at low, rather than at high partial pressure of oxygen. However, when self-oxidation of retinol is prevented, only a minor fraction of the antioxidant is allowed to effectively act as a radical scavenger, suggesting that the radical-trapping reactions are rate-limiting for the antioxidant process. Peroxyl radical concentration, by shifting the route of the retinol activity toward radical scavenging, brings about an increasingly more efficient radical trapping. It is concluded that all-trans retinol behaves as a more effective antioxidant at low oxygen partial pressure, low retinol concentrations, and high radical flux.

Reaction of melatonin with lipoperoxyl radicals in phospholipid bilayers

Melatonin, at 5 to 500 microM was incorporated in unilamellar soybean phosphatidylcholine (PC) liposomes, the peroxidation of which was induced by 2,2'-azobis (2-amidinopropane-hydrochloride) (AAPH), and measured as production of conjugated diene lipid hydroperoxides. Concentration as low as 5 and 10 microM were poorly effective in reducing lipid peroxidation. Melatonin at 30 to 500 microM caused short inhibition periods, increasing with, but not linearly related to concentration, with a concurrent net decrease of the propagation rate. The time course of melatonin oxidation, measured as loss of fluorescence, was studied during the AAPH-stimulated peroxidation of soybean PC liposomes, or when melatonin was incorporated in nonperoxidable unilamellar dimirystoyl phosphatidylcholine (DMC) liposomes. Consumption kinetics of 30 microM melatonin were linear with time in DMC liposomes and disappearance of melatonin occurred at a rate of 0.058 M(-8) s(-1). On the other hand, the consumption of melatonin during the oxidation of soybean PC liposomes, was not linear with time. The rate of disappearance was calculated as 0.19 M(-8) s(-1) at the beginning of the propagation phase, then it slowed down to reach the same rate observed in DMC liposomes. This evidence suggests a reaction with lipid-derived peroxyl radicals, possibly in addition to reaction with peroxyl radicals derived from AAPH. Scavenging of lipoperoxyl radicals by melatonin was also evident in experiments where melatonin was incorporated in multilamellar soybean PC liposomes and peroxidation was initiated by 2,2 '-azobis (2,4-dimethyl-valeronitrile). The antioxidant activity of melatonin in soybean PC liposomes is much lower than that of alpha-tocopherol, under comparable assay conditions. However, a combination of melatonin and alpha-tocopherol, at 5 microM, resulted in a synergistic antioxidant effect. Time course of alpha-tocopherol consumption, monitored in the absence and in the presence of melatonin, showed a significant decrease of the consumption rate when compounds were combined, indicating some protection by melatonin. Regeneration mechanisms were not evident and depletion of alpha-tocopherol was coincident with the inhibition time.

Oxidative stress and antioxidant status in beta-thalassemia major: iron overload and depletion of lipid-soluble antioxidants

Because of continuous blood transfusions, thalassemia patients are subjected to peroxidative tissue injury by the secondary iron overload. In accordance, analysis of serum from 42 beta-thalassemia patients, aged 4 to 40 years, showed that the mean concentrations of conjugated diene lipid hydroperoxides (CD), lipoperoxides evaluated as malondialdehyde/ thiobarbituric acid (MDA/TBA) adducts, and protein carbonyls increased about twofold with respect to control. Ferritin levels were positively correlated with the amount of MDA (r = .41; P = .007) and showed a positive trend with CD (r = .31; P = .07) and protein carbonyls (r = .35; P = .054), as further evidence of the deleterious effects of high tissue iron levels. Marked changes in the antioxidant pattern were also observed in all patients. Evidence is presented of a net drop in the concentration of ascorbate (-44%), vitamin E (-42%), vitamin A(-44%), beta-carotene (-29%), and lycopene (-67%). On the other hand, an increase of uric acid and bilirubin was observed, whereas serum albumin and glutathione were in the normal range in all patients. As a result, the total serum antioxidant potential, measured as trolox equivalent antioxidant capacity appeared significantly decreased by 14%. Serum levels of vitamin E were inversely correlated with ferritin (r = -.45; P = .003), suggesting a major consumption of this antioxidant under iron overload. Nontransferrin bound iron (NTBI) was in the range 4.5 to 54.8 micrograms/dL (mean, 21.8 +/- 13.9). Although NTBI had a positive trend with ferritin (r = .37, P = .03), no clear correlation was found with either MDA or vitamin E. A mild to severe hepatic damage, as assessed by serum transaminases, was shown in 24 of 42 patients. Serum levels of vitamin E (r = -.49, P = .015), vitamin A (r = -.48, P = .016) and lycopene (r = -.47, P = .020), were inversely correlated with the levels of transminases. On the other hand, lipid-soluble antioxidants in thalassemia patients were depleted to the same extent in hepatitis C virus (HCV)-infected (31 subjects) and in HCV-uninfected (10 subjects), while in the normal range in serum from 30 nonthalassemic patients with HCV-related chronic hepatitis. These results point out that the iron-induced liver damage in thalassemia may play a major role in the depletion of lipid-soluble antioxidants. The variations of the parameters evaluated in the present study were not correlated with the age of the patients. Our results suggest that the measurement of peroxidation products, matched with evaluation of antioxidants, may be a simple measure of iron toxicity in thalessemia, in addition to the conventional indices of iron status.

Synergistic interactions between vitamin A and vitamin E against lipid peroxidation in phosphatidylcholine liposomes

Interactions between alpha-tocopherol and all-trans retinol in suppressing lipid peroxidation were studied in a unilamellar liposomal system of phosphatidylcholine from either egg or soybean, in which peroxidation was initiated by the water-soluble azo initiator 2,2-azobis(2-amidino-propane)hydrochloride and peroxidation was measured as production of conjugated diene hydroperoxides. While all-trans retinol alone was poorly effective, the combination of all-trans retinol with alpha-tocopherol caused an inhibition period far beyond the sum of the inhibition periods observed with individual antioxidants, providing evidence of synergistic interactions. Furthermore, the inhibition rate calculated in the presence of both all-trans retinol and alpha-tocopherol, Rinh(E+A), was lower than Rinh(E) observed with alpha-tocopherol alone, suggesting that the extension of the inhibition time cannot be ascribed only to the antioxidant activity of alpha-tocopherol. The extent of synergism was linear with a molar ratio all-trans retinol/alpha-tocopherol ranging from 0.1 to 1.0, whereas a drop was observed at a ratio of 2.0. Synergistic antioxidant interactions between all-trans retinol and alpha-tocopherol were also evident when peroxidation was evaluated as production of malondialdehyde. A time course study, in which peroxidation of liposomes and depletion of antioxidants were concomitantly monitored, while showing that most of alpha-tocopherol was consumed to bring about the inhibition period, indicated that autooxidative reactions substantially contributed to the rapid depletion of all-trans retinol, when the antioxidants were allowed to act separately. On the other hand, when alpha-tocopherol and all-trans retinol were combined, the consumption of both antioxidants was significantly delayed, indicating reciprocal protection. Regeneration mechanisms cannot be accounted for by our results. The observed synergism between all-trans retinol and alpha-tocopherol does not appear as the result of specific structural interactions in the lipid bilayer. Combination of all-trans retinol with butylated hydroxytoluene, which reduced markedly all-trans retinol oxidation, resulted in a synergistic antioxidant activity greater than that observed with comparable amounts of alpha-tocopherol. In light of the known antioxidant mechanism of retinoids, the data suggest that by limiting autooxidation of all-trans retinol, alpha-tocopherol strongly promotes its antioxidant effectiveness. The concerted radical scavenging action in turn results in a synergistic protection of the lipid system against peroxidative stress and, ultimately, slows down the alpha-tocopherol consumption.

Reciprocal protective effects of all-trans retinol and alpha-tocopherol during lipid peroxidation in retinal membranes

Interactions between vitamin A and vitamin E in suppressing lipid peroxidation were observed in bovine retinal membrane preparations submitted to peroxidative injury by the water soluble azo initiator 2,2'-azobis(2-amidino-propane) hydrochloride (AAPH). Incorporation of 0.75 nmol mg prot(-1) all-trans retinol, an amount comparable with that of the endogenous alpha-tocopherol, significantly elongated the induction time preceding the release of TBA-reactive lipid peroxidation products, and reduced the consumption rate of the endogenous alpha-tocopherol. On the other hand, all-trans retinol was not able to induce any delay to the onset of lipid peroxidation when incorporated in membranes deprived of endogenous alpha-tocopherol by exposure to UV light, although TBARS produced within 60 min decreased slightly. Consumption of all-trans retinol during peroxidation was more rapid when all-trans retinol was incorporated in membranes deprived of alpha-tocopherol than in native membranes. These data suggest that reciprocal protective effects between vitamin A and vitamin E may strongly contribute to the defence of membranes against oxidative stress.

Contribution of vitamin A to the oxidation resistance of human low density lipoproteins

This study investigated the antioxidant contribution of vitamin A in protecting human low density lipoprotein (LDL) against copper-stimulated oxidation. The presence of small amounts of retinol (0.033 +/- 0.012 nmol/mol LDL) and retinyl palmitate (0.036 +/- 0.021 nmol/mol LDL) was routinely ascertained in the LDL. A single oral supplementation with 20,000 IU vitamin A caused a two- to three-fold increase of retinol and retinyl palmitate in the LDL isolated 8 h after the supplementation. In comparison to autologous-control LDL, vitamin A-enriched LDL were more resistant to oxidation, as expressed both by a clear delay in the onset of lipid peroxidation and by a reduction of the rate of conjugated diene hydroperoxide production during the propagation phase. The calculated incremental increase in the lag phase produced by 1 mol retinol per mol LDL is about 1000 min, suggesting that retinol is more potent than alpha-tocopherol in LDL. Oxidation experiments carried out with LDL isolated from plasma incubated in vitro with either retinol or retinyl palmitate indicated that retinol does lengthen the lag phase, whereas retinyl palmitate can slow the rate of peroxyl chain propagation, without affecting the duration of the lag phase. Temporal disappearance of retinol and retinyl palmitate, followed in comparison with that of alpha-tocopherol and beta-carotene, indicated that the reactivity of the antioxidants with lipoperoxyl radicals was in the sequence alpha-tocopherol, retinol, beta-carotene, and retinyl esters. Although the detailed antioxidant mechanism remains to be elucidated, these results suggest that LDL-associated vitamin A can play a role in maintaining the antioxidant status of LDL during oxidative stress in vivo.

Vitamin A preserves the cytotoxic activity of adriamycin while counteracting its peroxidative effects in human leukemic cells in vitro

Previous results from our laboratory gave evidence that safe doses of vitamin A were very effective in protecting rats from adriamycin-induced oxidative stress and lethal cardiotoxicity (Tesoriere, L. et al. (1994) J. Pharmacol. Experim. Ther. 269, 430-436). This was an incentive also to evaluate whether or not vitamin A affected the antitumor activity of adriamycin. K562 human erythroleukemia cells were exposed to adriamycin or to adriamycin plus vitamin A. Presence of 2.5 to 15 microM all-trans retinol in the cell culture did not impair the cytotoxicity of adriamycin. Rather, an enhanced cell death was observed when cell colony was exposed to both compounds. Additional assays showed that all-trans retinol counteracted the lipoperoxide formation, assayed as malondialdehyde, induced in cell cultures by the redox cycling activity of adriamycin. These data strongly encourage a new therapeuthical approach with safe doses of vitamin A as an adjuvant in cancer chemotherapy.

Effect of vitamin A administration on resistance of rat heart against doxorubicin-induced cardiotoxicity and lethality

The peroxyl radical-scavenging activity of vitamin A has been exploited to obtain protection against peroxidative damages induced in rat heart by administration of an acute dose of doxorubicin (10 mg/kg, in vein). Peroxidative lesions were evaluated by both biochemical and histological assays, 48 hr after the injection of doxorubicin. Heart tissue from rats receiving doxorubicin showed a marked increase of protein carbonyl levels, and of membrane conjugated dienes, as well as a decrease of membrane protein thiols. Abnormal chemistries, including a large increase of the activity of serum lactate dehydrogenase and creatine phosphokinase, an index of the myocardial damage caused by doxorubicin, were also observed. Pretreatment of rats with 25 I.U./kg b.wt. of vitamin A, once a day for 2 days, before injecting doxorubicin, substantially reduced the peroxidative damage to heart lipids and proteins, and markedly lowered the serum values of lactate dehydrogenase and creatine phosphokinase to values close to those of control rats. The significant prevention of doxorubicin-induced cardiomyopathy by vitamin A was evident from the histopathological pattern observed after light microscopy. The dosage of vitamin A useful to obtain the protective effect appears safe and does not injure the liver, as indicated by light microscopy of the tissue. A survival study, carried out by injecting rats with a single injection of 10 mg/kg of doxorubicin, showed that pretreatment with 25 I.U. of vitamin A per kg significantly increased survival rate of the animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Antioxidant activity of all-trans-retinol in homogeneous solution and in phosphatidylcholine liposomes

A kinetic quantification of the lipoperoxyl radical-scavenging activity of all-trans-retinol has been carried out in homogeneous solution, when radicals were produced from the oxidation of methyl linoleate in methanol, initiated by the lipid-soluble 2,2'-azobis (2,4-dimethylvaleronitrile) (AMVN) as well as in a soybean phosphatidylcholine membrane model, in which peroxidation was induced either by AMVN or the hydrophylic 2,2'-azobis(2-amidinopropane)hydrochloride (AAPH). The physical microenvironment contributes to the determination of antioxidant efficiency of all-trans-retinol. In homogeneous solution the kinetic constant kinh is 3.5 x 10(5) M-1 s-1 and appears of the same order of magnitude as the inhibition constant measured for alpha-tocopherol under the same experimental conditions. Nevertheless, despite its very high chemical reactivity toward lipoperoxyl radicals, the overall antioxidant efficiency of all-trans-retinol in this system appears quite limited, since the evaluated stoichiometric factor is 0.21. When the polyenoic chain of all-trans-retinol is incorporated into a phosphatidylcholine lipid bilayer, the antioxidant efficiency depends on the site of peroxyl-radical production. The highest lipoperoxyl radical-scavenging activity is measured when radicals are generated by AHVN inside the bilayer multilamellar liposomes. Under these conditions, the relative antioxidant efficiency is similar to that of alpha-tocopherol, and the stoichiometric factor is 3.1. When radicals are generated by AAPH in the aqueous phase of an unilamellar liposomal system, the antioxidant effectiveness of all-trans-retinol appears reduced and lower than that measured with equivalent amounts of alpha-tocopherol. Synergistic antioxidant effects between all-trans-retinol and alpha-tocopherol are observed when both antioxidants are simultaneously incorporated into unilamellar liposomes in which peroxidation is induced by AAPH. This suggests that all-trans-retinol may interact with tocopheroxyl radicals, thereby regenerating alpha-tocopherol. This interaction, which may be related to molecular features and to the relative location of the antioxidants in the bilayer, could provide an effective antioxidant system that may be of great importance in vivo.

Vitamin A inhibits doxorubicin-induced membrane lipid peroxidation in rat tissues in vivo

The antioxidant activity of vitamin A against lipid peroxidation induced by doxorubicin in rat tissues in vivo was investigated. A single ip injection of doxorubicin (30 mg/kg body wt) markedly raised the level of peroxidated lipids measured as TBARS and conjugated dienes in heart and brain membrane preparations. Other tissues, such as retina and liver, did not show any increase of lipid peroxides over control values. Pretreatment of rats with two daily subcutaneous injections of retinol palmitate (0.25 g/kg body wt), for 2 days, before injecting doxorubicin, inhibited peroxidation of heart and brain membrane lipids. The antioxidant action of vitamin A does not appear to be mediated by enhancement of antioxidant enzyme activities committed to detoxify oxygen radicals. Superoxide dismutase and catalase, measured in heart and brain cytosol, were not affected by the vitamin A treatment. On the contrary, a slight increase of catalase activity was observed in heart and brain cytosol from rats that had been treated with doxorubicin. Excess vitamin A may be localized in membranes. Appreciable increase of retinyl esters and retinol was measured in membrane preparations from rats that had been treated with vitamin A, with respect to control animals. Brain and heart membrane preparations from rats receiving vitamin A, assayed in vitro in the presence of an Fe3+ ascorbate induction system, showed a delay at the beginning of the lipid peroxidation and generated lesser amounts of TBARS, with respect to membranes from control rats. Thus, the increase of vitamin A within cell membranes results in an increased resistance of membrane lipids to peroxidation, both endogenously produced and induced in vitro. These results may be consistent with the hypothesis that vitamin A may act as a physiological antioxidant in cell membranes where it is localized.

All-trans to 11-cis retinol isomerization in nuclear membrane fraction from bovine retinal pigment epithelium

Isomerization of all-trans to 11-cis retinol has been studied in a membrane preparation from the nuclear fraction of bovine retinal pigment epithelium. When the nuclear membrane preparation deprived of endogenous retinoids is incubated with 4.5 microM all-trans-retinol, the mean value calculated for the isomerase activity is 1.32 nmol 11-cis retinol formed hr-1 mg protein-1. Simultaneous formation of all-trans and 11-cis retinyl esters is also observed in the nuclear preparation. When assayed under the same experimental condition, RPE 150,000 g post-nuclear sediment shows about 70% of the isomerase activity found in the nuclear membrane fraction. Treatment of the nuclear membrane fraction with 0.5% (w/v) CHAPS produces a 200,000-g supernatant retaining 80% of the total isomerase activity and leads to a modest purification of the enzyme activity. Apparent values for Km and Vmax of the solubilized enzyme are 1.6 microM and 2.5 nmol 11-cis retinol formed h-1 mg protein-1, respectively. Bovine serum albumin and beta-lactoglobulin effectively stimulate the isomerization reaction. The mechanism underlying this activating effect remains unclarified at present. Some hypotheses are discussed.

Distribution of vitamin A compounds in bovine eyes after bleaching adaptation

A seasonal increase in the amount of bleached rhodopsin caused, in living animals, by the seasonal increase of the intensity of sunlight in the early morning before the calves are killed, was verified in the bovine eyes subjected to the present study. This was used as a means of assaying distribution and isomer composition of esterified and unesterified retinol in eyes from animals light-adapted to a different extent under environmental conditions. The progressive increase of bleached rhodopsin results in a parallel increase of all-trans-retinol in retina and of both all-trans- and 11-cis-retinyl esters in pigment epithelium. Analytical subcellular fractionation of RPE homogenate reveals that retinyl esters accumulate without an exclusive subcellular localization in nuclear, mitochondrial/lysosomal and microsomal fractions. Whatever the amount of bleached rhodopsin, only small and constant amounts of retinyl esters are found in the soluble fraction of RPE, entirely under the all-trans configuration. When a considerable portion of rhodopsin is bleached (about 70%), substantial amounts of all-trans-retinol, along with minor amounts of 11-cis-retinol, accumulate in RPE subcellular organelles. The in vitro bleaching of bovine eyes results in a distribution of retinoids between retina and RPE which appears different from that detected in eyes naturally bleached to the same extent.

Analysis of a soluble lipid-protein complex carrying endogenous 11-cis retinaldehyde from bovine retinal pigment epithelium

A soluble lipid-protein complex in bovine retinal pigment epithelium is shown to carry endogenous 11-cis retinaldehyde, in the extent of 15% of the total 11-cis retinaldehyde found in this tissue. The complex, analyzed with respect to its chemical composition, exhibits a lipid composition close resembling the lipid composition of the rod outer segment membrane; the SDS-PAGE evidences the presence of a number of protein bands, two of which of 34 and 27 kDa appear glycoproteins. Finally, the lipid-protein complex exhibits a discrete level of a Cathepsin D-like protease activity. From the above, the possibility is discussed that the soluble lipid-protein complex could represent some phagolysosomal inclusion occurring in the pigment epithelial cells upon rod outer segment phagocytosis.

Modulation by docosahexaenoic acid of the epinephrine-stimulated adenylate cyclase activity of the bovine retina

This work shows that unsaturated fatty acids enhance the epinephrine-stimulated adenylate cyclase activity in bovine retina. The modulating effect on the epinephrine-stimulated formation of cyclic AMP seems to be linked to the degree of unsaturation of the fatty acid. Treatment of the intact retina with docosahexaenoic acid in the concentration range 0.5 X 10(-6)-1 X 10(-3) M does not affect the enzyme activity measured in the absence of the hormone but markedly increases the cyclase activity when the tissue is incubated in the presence of 0.1 mM epinephrine. Docosahexaenoic acid enhances the maximal response to epinephrine without affecting the apparent ED50 value for this effector. Docosahexaenoic acid at 0.5 mM also increases the hormone-stimulated adenylate cyclase activity in retinal cell-free homogenate, whereas it has no effect on the epinephrine-sensitive enzyme solubilized from the membrane fraction with 1% Triton X-305. When docosahexaenoic acid-preincubated intact retina and cell-free homogenate are incubated in the presence of defatted albumin, both the observed activating effect of the fatty acid on the epinephrine-stimulated adenylate cyclase activity and the enhancement of the enzyme response to the hormone significantly diminish.

Binding of 11-cis retinaldehyde to the partially purified cellular retinaldehyde binding protein from bovine retinal pigment epithelium

11-cis retinaldehyde binding analysis was performed on a bovine retinal pigment epithelium preparation of cellular retinaldehyde binding protein (CRALBP), whose purity degree was estimated as 75%. Equilibrium binding studies were carried out measuring the replacement of tritium-labeled with unlabeled 11-cis retinaldehyde at 25 degrees C. Analysis of the experimental data both by a direct curve-fitting procedure utilizing a non linear least square regression analysis and by a conventional Scatchard plot revealed a single non-interacting binding site with an apparent equilibrium constant of 0.9 X 10(-7) M. A binding stoichiometry of approximately 1 mol of 11-cis retinaldehyde/mol of binding protein can be calculated from the experimental data. Competition studies carried out in the presence of unlabeled 'trans' and 'cis' isomers of vitamin A derivatives confirm the high degree of specificity of the 11-cis retinaldehyde binding.

Nucleoside phosphotransferase in animal tissues. Tissue distribution and kinetic properties

Amphibian, avian and mammal tissues contain a nucleoside phosphotransferase clearly different from those previously described in vegetables and bacteria. Whatever the animal source, the enzyme showed many similar characteristics as far as substrate specificity, dependence upon Mg2+, instability at 37 degrees C, and the protecting effect of nucleotides were concerned. Moreover, when submitted to gel filtration, the enzyme behaved in all cases as a dissociable high molecular weight protein, whose degree of association was controlled by nucleotides. In amphibian and avian tissues multiple forms of the enzyme seem to be present which differ for the substrate concentration at half-maximal velocity (S0.5); the concentration of nucleotide effector which affords half-maximal protection at 37 degrees C (P0.5); and the Hill coefficient for monophosphate donor. Within each single species, the higher the interaction coefficient was, the lower S0.5 and P0.5 values were. In mammalian tissues one form of nucleoside phosphotransferase seems to prevail where cooperative interactions are almost absent and whose S0.5 as well as P0.5 values do not vary significantly from one tissue to another.

The inhibitory effect of D-glucosamine on thymidine kinase in chick embryo retinas and HeLa cells

D-Glucosamine markedly inhibits thymidine incorporation into the TCA-insoluble fraction and thymidine kinase activity in HeLa cells. Both the inhibitory effects are also observed in isolated retinas of chick embryos. In this case the inhibitory effects are age-dependent and the magnitude of the responses decreases with embryonic development. In addition the time of exposure to D-glucosamine which is necessary to reveal the inhibitory effect on thymidine kinase increases with the age of the embryos.

The purification and properties of nucleoside phosphotransferase from mucosa of chicken intestine

Nucleoside phosphotransferase (nucleotide: 3'-deoxynucleoside 5'-phosphotransferase, EC 2.7.1.77) has been purified from chicken intestine mucosa to apparent homogeneity. The enzyme is represented by a multisubunit protein at different degrees of association. It can dissociate into a component with a marked fall in catalytic activity. The associated forms are similar to the enzyme previously purified from chick embryo as regards: substrate specificity both with respect to nucleoside monophosphate donors and to deoxyribonucleoside acceptors; sigmoidicity in the rate curve with a variable phosphate donor; instability to heat, dilution and lowering of pH; the activating and protecting effect of nucleotides, particularly the diphosphate forms. The dissociated form displays lower Vmax and higher S0.5 than the associated ones; and the Hill constants are always about 1. With this form, nucleotides show only a modest activating effect and do not protect. Mg2+, Mn2+ or Co2+ are required for catalytic activity, whereas the protective effect of nucleotides is independent of divalent metals. Inorganic phosphate stabilizes associated forms of the enzyme, but inhibits its activity by competing with nucleotide effectors. The enzyme behaves also as a phosphohydrolase, particularly with respect to deoxyribonucleoside monophosphates; deoxyuridine and deoxythymidine inhibit hydrolytic activity.

The effects of glucocorticoids on thymidine kinase and nucleoside phosphotransferase during development of chicken embryo retina

Thymidine kinase in chick embryo retina reaches its highest values on the 8-10th day of development, then declines reaching the lowest value at hatching. The rate of DNA synthesis essentially follows this activity while, in contrast, nucleoside phosphotransferase increases progressively during development. Glucocorticoids at 5 X 10(-6) M lower the level of thymidine kinase in isolated retinas of chick embryo. The most effective steroid was hydrocortisone. The effect was observed in retinas from 8-18-day-old chick embryo and, except on the 8th day, was always of the same magnitude. We suggest that a glucocorticoid can be the natural factor responsible for the marked fall in thymidine kinase during development. Brief periods of exposure to steroids increase nucleoside phosphotransferase activity in isolated chick embryo retinas. When the exposure was longer than 3 h this activity was also clearly decreased. We conclude that other factors are responsible for the natural increment which occurs for this activity during development.

Development of nucleoside phosphotransferase activity in the cerebral hemispheres of embryonal and adult chick

In the cerebral hemispheres of the chick embryo, the level of nucleoside phosphotransferase activity is much higher than that of thymidine kinase and it increases progressively during development up to the adult stage. Therefore nucleoside phosphotransferase is not coupled with DNA synthesis.