beta-carotene and azobis(isobutyronitrile)

beta-Carotene and other xanthophylls present in pepper fruit as both free and esterified forms were oxidized using a free radical initiator (2,2'-azo-bis-isobutyronitrile). Capsorubin was degraded most slowly, followed by zeaxanthin, capsanthin, and beta-carotene. The presence of keto groups at the ends of the polyene chain could be a structural factor contributing to this difference in reactivity. It was also shown that whereas capsanthin and its esters and capsorubin and its esters were degraded at the same rate, zeaxanthin esters responded differently to the oxidation process, and were degraded more quickly than free zeaxanthin. The presence of unsaturated fatty acids (mainly linoleic) that esterify zeaxanthin help to accelerate the degradation of this xanthophyll and decreasing its antioxidant action. The antioxidant capacity of capsorubin and capsanthin (both in free and esterified form) exclusive to the genus capsicum should be taken into account.

Topics: Antioxidants; beta Carotene; Carotenoids; Esterification; Free Radicals; Nitriles; Oxidation-Reduction; Xanthophylls; Zeaxanthins

The effect of oxygen partial pressure and solvent on the kinetics of beta-carotene consumption and oxygen uptake in the course of beta-carotene oxidation has been studied. The reaction of free radical formation, i.e., the chain initiation reaction that occurs via interaction of beta-carotene with oxygen and chain branching reactions that occur via interaction of intermediate products with oxygen, are found to be the most sensitive to the effect of environmental factors. These reactions determine the dependence of the beta-carotene consumption and oxygen uptake rates on oxygen partial pressure. The reaction rates increase in polar solvents which causes the increase in the beta-carotene consumption and oxygen uptake rates. The rates of peroxyl radical destruction and/or isomerization increase in polar media, which leads to the formation of low molecular weight polar oxidation products.

Topics: Alkanes; beta Carotene; Chlorobenzenes; Cyclohexenes; Free Radicals; Kinetics; Limonene; Lipids; Models, Chemical; Nitriles; Oxygen; Partial Pressure; Terpenes

The effect of fatty acid unsaturation on the antioxidant ability of beta-carotene and alpha-tocopherol to inhibit azobis-isobutyronitrile AIBN)-induced malondialdehyde (MDA) formation is investigated in a hexane solution. A positive correlation is shown between the fatty acid unsaturation and MDA production in homogeneous solutions. Both beta-carotene and alpha-tocopherol act as chain-breaking antioxidants in our model, effectively suppressing AIBN-induced MDA formation. When alpha-tocopherol is added to fatty acid solutions, a lag phase of about 30 min is observed and a propagation phase is produced at a rate dependent on the degree of unsaturation and similar to that observed in the absence of the antioxidant. A specific inhibition of initiation phase by alpha-tocopherol is confirmed by its total consumption after 30 min of incubation with AIBN. On the other hand, when beta-carotene is added, a lag period is not observed and the inhibition of propagation phase progressively increases in relation to the degree of fatty acid unsaturation. These data present different antioxidant roles for beta-carotene and alpha-tocopherol in AIBN-induced lipid peroxidation and suggest that beta-carotene can be a very effective antioxidant in highly unsaturated membranes, such as those enriched with n-3 polyunsaturated fatty acids (PUFA).

Topics: Analysis of Variance; Antioxidants; beta Carotene; Carotenoids; Fatty Acids, Unsaturated; Free Radicals; Hexanes; Kinetics; Malondialdehyde; Nitriles; Vitamin E

Carotenoid scavenging of free radicals has been investigated in peroxidizing methyl esters of unsaturated fatty acids using (i) metmyoglobin as a water-based free-radical initiator in a heterogeneous lipid/water system, and (ii) azo-bis-isobutyronitrile as a free-radical initiator in a homogeneous chloroform solution. For the heterogeneous system, using a combination of electrochemical oxygen depletion measurements, spectrophotometric determination of lipid hydroperoxides and carotenoid degradation, it was demonstrated that each of the four carotenoids astaxanthin, beta-carotene, canthaxanthin, and zeaxanthin protects the methyl esters against oxidation. The antioxidative effect increases with increasing carotenoid concentration, increases with decreasing oxygen partial pressure (0.010 < pO2 < 0.50 atm), and shows little dependence on the structure of the carotenoid. For a homogeneous solution, the effect of the structure of the carotenoid was further investigated, and it was shown that the stability of the four carotenoids in the oxidizing system are different, with the order of decreasing stability being: astaxanthin > canthaxanthin > beta-carotene > zeaxanthin. Each of the four carotenoids can suppress lipid oxidation and the degree of suppression of peroxidation of methyl linoleate corresponds to the difference in stability.

Topics: Adipose Tissue; beta Carotene; Canthaxanthin; Carotenoids; Free Radical Scavengers; Kinetics; Linoleic Acids; Linolenic Acids; Lipid Metabolism; Metmyoglobin; Models, Biological; Nitriles; Oxidation-Reduction; Oxygen; Oxygen Consumption; Structure-Activity Relationship; Xanthophylls; Zeaxanthins

Rat liver microsomal lipids in hexane solution were exposed to the lipid-soluble radical initiator, azobis-isobutyronitrile (AIBN), and the antioxidant activities of alpha-tocopherol and beta-carotene have been compared. Lipid peroxidation was monitored both by conjugated diene formation at 233 nm, and by malondialdehyde (MDA) formation in the thiobarbituric acid assay at 535 nm. Diene formation was continuous for at least 120 min in the presence of 85 micrograms/ml lipid and 4 mM AIBN. Both alpha-tocopherol and beta-carotene acted as chain-breaking antioxidants, suppressing lipid peroxidation and producing an induction period at concentrations as low as 0.5 and 8 microM, respectively. When both of these lipid-soluble antioxidants were present together, the oxidation was strongly suppressed and the induction period was the sum of the individual antioxidants, alpha-Tocopherol and beta-carotene also inhibited MDA generation. In the presence of 170 micrograms/ml lipid and 8 mM AIBN, beta-carotene exhibited an IC50 of 1.1 microM and inhibited completely at 15 microM. Using beta-carotene, an induction period was observed, although much less pronounced than with alpha-tocopherol. Furthermore, beta-carotene inhibited MDA production in a concentration-dependent manner and exhibited an IC50 of 50 microM. In addition, added beta-carotene delayed the radical-initiated destruction of the endogenous alpha-tocopherol and gamma-tocopherol in this system.

Topics: Animals; beta Carotene; Carotenoids; Dose-Response Relationship, Drug; Free Radicals; Kinetics; Lipid Peroxidation; Malondialdehyde; Microsomes, Liver; Nitriles; Rats; Rats, Inbred Strains; Vitamin E