Peter T. Southwell-Keely

Is α-tocopherol a reservoir for α-tocopheryl hydroquinone?

The products of oxidation of the alpha-tocopherol model compound, 2,2,5,7,8-pentamethyl-6-chromanol (PH) by t-butyl hydroperoxide in chloroform varied with the amount of water present. In the presence of a trace of water, the main products were the spirodimer (PSD) and spirotrimer (PST). As the content of water increased, the main product became 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (PQ). Oxidation of PH in aqueous liposome suspension also produced PQ as the major product. These results suggested that, in aqueous solutions, the major oxidation product of PH would be PQ and of alpha-tocopherol (TH) would be alpha-tocopheryl quinone (TQ). The ease of reduction of PQ and TQ was studied in chemical and biological systems. PQ, TQ, and ubiquinone-10 (UQ) were rapidly reduced to their respective hydroquinones (PQH2, TQH2, and UQH2) at pH 7.3 by NADH plus FAD. Whole blood reduced PQ rapidly at 37 degrees C to PQH2 but did not reduce TQ to TQH2. Human peripheral blood mononuclear cells took up TQ from a bovine serum albumin complex and reduced it to TQH2. Ingestion of TQ (350 mg) by one of us (PSK) resulted in the formation of TQH2 during a 5 h period. These results demonstrate that several biological systems are able to reduce TQ to TQH2 and that it is a reaction that may occur normally in vivo.

Cytotoxicity of tocopherols and their quinones in drug-sensitive and multidrug-resistant leukemia cells

Cytotoxicities of tocopherols (α-T, γ-T, δ-T), their para (α-TQ, γ-TQ, δ-TQ)-and ortho (Tocored)-quinone oxidation products, the synthetic quinone analog of γ-TQ containing a methyl group substituted for the phytyl side-chain (TMCQ) and the synthetic quinone analog of Tocored containing a methyl group substituted for the phytyl side-chain (PR) were measured in acute lymphoblastic leukemia cell lines that are drug-sensitive (CEM) and multidrug-resistant (CEM/VLB100). Among tocopherols, only δ-T exhibited cytotoxicity. Among para quinones, α-TQ showed no cytotoxicity, while γ-TQ and δ-TQ were highly cytotoxic in both CEM and CEM/VLB100 cell lines (LD50<10 μM). δ-TQ and γ-TQ were more cytotoxic than the widely studied chemotherapeutic agent doxorubicin, which also showed selective cytotoxicity to CEM cells. The orthoquinone Tocored was less cytotoxic than doxorubicin in drug-sensitive cells but more cytotoxic than doxorubicin in multidrug-resistant cells. Cytotoxicity was not a function of the phytyl side-chain since both TMCQ and PR were cytotoxic in leukemia cells. Cytotoxic para and ortho quinones were electrophiles that formed adducts with nucleophilic thiol groups in glutathione and 2-mercaptoethanol. Cytotoxicity was enhanced when the glutathione pool was depleted by preincubation with buthionine-[S,R]-sulfoximine, but cytotoxicity was diminished by the addition of N-acetylcysteine to cultures. α-T also diminished the cytotoxicity of para- and or-thoquinones. Buthionine-[S,R]-sulfoximine did not block the inhibitory effect of either N-acetylcysteine or α-T, showing that these agents did not act solely by maintaining the glutathione pool as an essential antioxidant system. In conclusion, tocopherylquinones represent a new class of alkylating electrophilic quinones that function as highly cytotoxic agents and escape multidrug resistance in acute lymphoblastic leukemia cell lines.

Probucol Protects against Hypochlorite-induced Endothelial Dysfunction IDENTIFICATION OF A NOVEL PATHWAY OF PROBUCOL OXIDATION TO A BIOLOGICALLY ACTIVE INTERMEDIATE

Atherosclerosis is associated with endothelial dysfunction and a heightened state of inflammation characterized, in part, by an increase in vascular myeloperoxidase and proteins modified by its principal oxidant, hypochlorous acid (HOCl). Here we examined whether probucol could protect against endothelial dysfunction induced by the two-electron oxidant HOCl. Hypochlorous acid eliminated endothelium-dependent relaxation of rabbit aorta, whereas endothelial function and tissue cGMP was preserved and elevated, respectively, in animals pretreated with probucol. Exogenously added probucol also protected against HOCl-induced endothelial dysfunction. In vitro, HOCl oxidized probucol in a two-phase process with rate constants k1 = 2.7 ± 0.3 × 102 and k2 = 0.7 ± 0.2 × 102 m–1 s–1 that resulted in a dose- and time-dependent accumulation of probucol-derived disulfoxide, 4,4′-dithiobis(2,6-di-tert-butyl-phenol) (DTBP), DTBP-derived thiosulfonate, disulfone, and sulfonic acid, together with 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) as determined by high performance liquid chromatography and mass spectrometry. Like HOCl, selected one-electron oxidants converted probucol into DTBP and DPQ. Also, dietary and in vitro added DTBP protected aortic rings from HOCl-induced endothelial dysfunction and in vitro oxidation by HOCl gave rise to the thiosulfonate, disulfone, and sulfonic acid intermediates and DPQ. However, the product profiles of the in vitro oxidation systems were different from those in aortas of rabbits receiving dietary probucol or DTBP ± HOCl treatment. Together, the results show that both probucol and DTBP react with HOCl and protect against HOCl-induced endothelial dysfunction, although direct scavenging of HOCl is unlikely to be responsible for the vascular protection by the two compounds.

Further oxidation products of 2,2,5,7,8-pentamethyl-6-chromanol

In order to undertake a quantitative study by high-performance liquid chromatography of the rate of oxidation of 2,2,5,7,8-pentamethyl-6-chromanol (1), the model compound of α-tocopherol, a number of potential products were required as standards. Among these compounds were 2,2,7,8-tetramethylchroman-5,6-dione (10) and 2,2,7-trimethyl-6-hydroxychroman-5,8-dione (17), the model compounds of tocored and tocopurple, respectively. Attempts to synthesize 10 and 17 led to the isolation of 8-hydroxymethyl-2,2,7-trimethylchroman-5,6-dione (14) and 1,2-bis(2,2,7-trimethylchroman-5,6-dione-8-)ethane (19) a dimer of 10. Purification by thin-layer chromatography of the spirodimer (20) of 1 resulted in an acid-catalyzed decomposition to 1-(2,2,7,8-tetramethyl-6-chromanol-5-)2-[2-(3-methyl-3-hydroxybutyl)-5,6-dimethyl-1,4-benzoquinone-3-]ethane (23), a new chromanol-quinone dimer.

Antioxidant activity of oxidation products of α-tocopherol and of its model compound 2,2,5,7,8-pentamethyl-6-chromanol

A variety of oxidation products (4–29) of α-tocopherol, 1 and of its model compound, 2,2,5,7,8-pentamethyl-6-chromanol (2) has been tested for antioxidant activity against autoxidizing safflower oil (ASO) and autoxidizing methyl linoleate (AML). The following compounds showed good antioxidant activity against both substrates: 5-hydroxymethyl-2,2,7,8-tetramethyl-6-chromanol (4), 5-(2,2,5,7,8-pentamethyl-6-chromanoxy)methyl-2,2,7,8-tetramethyl-6-chromanol (15), 1,2-bis(2,2,7,8-tetramethyl-6-chromanol-5-)ethane (16), 5-ethoxymethyl-7,8-dimethyltocol (19), 5-mthoxymethyl-2,2,7,8-tetramethyl-6-chromanol (21), 5-ethoxymethyl-2,2,7,8-tetramethyl-6-chromanol (20), 5-propoxymethyl-2,2,7,8-tetramethyl-6-chromanol (22), 5-butoxymethyl-2,2,7,8-tetramethyl-6-chromanol (23), 5-(2-methyl-1-propoxy)methyl-2,2,7,8-tetramethyl-6-chromanol (24), 5-(2-methyl-2-propoxy)methyl-2,2,7,8-tetramethyl-6-chromanol (25), 5-heptoxymethyl-2,2,7,8-tetramethyl-6-chromanol (26), 5-undecoxymethyl-2,2,7,8-tetramethyl-6-chromanol (27), 5-phytoxymethyl-2,2,7,8-tetramethyl-6-chromanol (28) and 5-cholesteroxymethyl-2,2,7,8-tetramethyl-6-chromanol (29). 2,2,7,8-Tetramethylchroman-5,6-dione (17) and 1,2-bis(2,2,7-trimethylchroman-5,6-dione-8-)ethane (18) showed significant antioxidant activity against ASO but not against AML. If the corresponding oxidation products of 1 are formedin vivo it means that the antioxidant activity of 1 is not lost on oxidation. This may help to explain the outstanding capacity of 1 to protect cell membranes.

Effect of alcohols on the oxidation of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol

The vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, has been oxidized witht-butyl hydroperoxide in chloroform in order to simulate in vivo oxidations due to lipid hydroperoxides. In the presence of a variety of alcohols, ranging from methanol to cholesterol, the corresponding 5-alkoxymethyl-2,2,7,8-tetramethyl-6-chromanols were formed in fair to good yield and were the major products in each reaction.

Oxidation of the α-tocopherol model compound 2,2,5,7,8-pentamethyl-6-chromanol in the presence of alcohols.

Oxidation of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol (1b) byt-butyl hydroperoxide in chloroform has been studied in the presence of ethanol, heptanol and cholesterol. In the absence of an alcohol, the major products were the spirodimer (13b) and spirotrimer (14b) of 1b, together with 1H,2,3-dihydro-3,3,5,6,9,10,11a(R)-heptamethyl-7a(S)-(3-hydroxy-3-methylbutyl)-pyrano[2,3-a] xanthene 8(7aH), 11(11aH) dione (6b). In the presence of ethanol, heptanol and cholesterol, the major products were 5-ethoxymethyl-2,2,7,8-tetramethyl-6-chromanol (16b), 5-heptoxymethyl-2,2,7,8-tetramethyl-6-chromanol (17) and 5-cholesteroxymethyl-2,2,7,8-tetramethyl-6-chromanol (18). However, when water was present in a homogeneous reaction, the most rapidly formed product was 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (5b). Compounds 13b, 14b, 16b, 17 and 18 are formedvia a quinone methide intermediate, and compound 5b is formedvia a phenoxylium ion. The phenoxylium species appears to be the preferred intermediate when water is present, whereas the quinone methide species is prefered in the absence of water.

New oxidation products of α-tocopherol

A major product of the reaction between α-tocopherol andt-butyl hydroperoxide in chloroform is 5-ethoxymethyl-7,8-dimethyl tocol, the source of the ethoxy group being the ethanol that is used to stabilize the chloroform. Two new products of this oxidation have now been identified as 2-(3′-hydroxy-3′,7′,11′,15′-tetramethylhexadecyl), 3-ethoxymethyl-5,6-dimethylbenzo-1,4-quinone and 5-ethoxycarbonyl-7,8-dimethyl tocol. These two compounds and another major product, 5-formyl-7,8-dimethyl tocol appear to be formed by further oxidation of 5-ethoxymethyl-7,8-dimethyl tocol.

Separation of the α-Tocopherol Model Compound 2,2,5,7,8-Pentamethyl-6-Chromanol from its Oxidation Products by High Performance Liquid Chromatography

Abstract A rapid method is described for the separation of the α-tocopherol model compound, 2,2,5,7,8-pentamethyl-6-chromanol (6), from 9 of its oxidation products in a single 35 minute run. Separated derivatives of 6, in order of elution, included the 5-cholesteroxymethyl (1), spirotrimer (2), spirodimer (3), 5-formyl (4), 5-ethoxymethyl (5), dihydroxydimer (7), chroman dione (8), quinone (9) and pyrano xanthene (10). A normal phase system, using gradient elution is employed, the eluent being monitored at 290 nm. The minimum detection limit for compounds 1–8 was 0.1 μg per injection and for compounds 9 and 10 it was 0.3 μg per injection.

Oxidations of the α-tocopherol model compound 2,2,5,7,8-pentamethyl-6-chromanol. Formation of 2,2,7,8-tetramethylchroman-5,6-dione

Overoxidation of α-tocopherol (1a) by silver nitrate produces tocored (9a) as a major product. The aim of the present work was to elucidate the pathway of formation of tocored using the α-tocopherol model compound, 2,2,5,7,8-pentamethyl-6-chromanol (1b). Oxidation of 1b by silver nitrate in ethanol produces 2-(3-hydroxy-3-methylbutyl)-3,5,6-trimethyl-1,4-benzoquinone (6b) and 2,2,7,8-tetramethylchroman-5,6-dione (9b, the model compound of tocored) as major products. Formation of 6b is rapid and is accompanied by an equally rapid fall in pH. Formation of 9b only occurs after 6b has reached maximum concentration and has begun to decline. It appears that acid promotes the dehydration and recyclization of 6b into a quinone methide (2b), which is then rehydrated into 5-hydroxymethyl-2,2,7,8-tetramethyl-6-chromanol (5b), the phenolic isomer of the quinone 6b. Oxidative deformylation of 5b leads to 9b. It is also demonstrated that 6b, heated in ethanol in the presence of acid and in the absence of any oxidizing agent, is converted into 9b, 1b, 5-ethoxymethyl-2,2,7,8-tetramethyl-6-chromanol (4b) and 2-(3-hydroxy-3-methylbutyl)-3-ethoxymethyl-5,6-dimethyl-1,4-benzoquinone (7b). It seems that dehydration and recyclization of 6b into 5b occurs as above and that 6b then oxidizes 5b into 9b, while being reduced into the hydroquinone of 6b (6bH2). Compound 6bH2 then cyclizes in acid to 1b. A possible alternative pathway from 6b to 9b that does not involve 5b is also discussed. These results suggest that 6b and, by implication, α-tocopheryl quinone (6a), is not a stable compound and, in the presence of acid, is readily oxidized to 9b.