Inhibition of human glutathione S-transferase P1-1 by tocopherols and alpha-tocopherol derivatives.

alpha-Tocopherol inhibits glutathione S-transferase P1-1 (GST P1-1) (R.I.M. van Haaften, C.T.A. Evelo, G.R.M.M. Haenen, A. Bast, Biochem. Biophys. Res. Commun. 280 (2001)). In various cosmetic and dietary products alpha-tocopherol is added as a tocopherol ester. Therefore we have studied the effect of various tocopherol derivatives on GST P1-1 activity. It was found that GST P1-1 is inhibited, in a concentration dependent manner, by these compounds. Of the compounds tested, the tocopherols were the most potent inhibitors of GST P1-1; the concentration giving 50% inhibition (IC(50)) is <1 microM. The esterified tocopherols and alpha-tocopherol quinone also inhibit the GST P1-1 activity at a very low concentration: for most compounds the IC(50) was below 10 microM. RRR-alpha-Tocopherol acetate lowered the V(max) values, but did not affect the K(m) for either 1-chloro-2,4-dinitrobenzene or GSH. This indicates that the GST P1-1 enzyme is non-competitively inhibited by RRR-alpha-tocopherol acetate. The potential implications of GST P1-1 inhibition by tocopherol and alpha-tocopherol derivatives are discussed.

No reduction of alpha-tocopherol quinone by glutathione in rat liver microsomes.

The cell membrane is protected against lipid peroxidation by endogenous antioxidants such as vitamin E (alpha-tocopherol). The oxidised form of alpha-tocopherol (alpha-tocopherol quinone) does not have this antioxidant function. However, the literature indicates that alpha-tocopherol quinone can be reduced to alpha-tocopherol in vivo and thereby will add to the total antioxidant potential (Moore AN, Ingold KU. Free Radic Biol Med 1997;22:931-4). We found that GSH (reduced glutathione) did not mediate the reduction of alpha-tocopherol quinone, either directly in solution or in rat liver microsomes fortified with alpha-tocopherol quinone. This renders GSH a less likely candidate for alpha-tocopherol quinone reduction in vivo. In addition, alpha-tocopherol quinone did not enhance GSH-dependent protection against lipid peroxidation, either in control microsomes, or in vitamin E-extracted microsomes. Indeed, alpha-tocopherol quinone blocked GSH-dependent protection against lipid peroxidation in vitamin E-extracted microsomes. This indicates that alpha-tocopherol quinone can act as a pro-oxidant.

Hypochlorous acid is a potent inhibitor of GST P1-1.

Glutathione S-transferase is a phase II detoxification enzyme that can be inactivated by H(2)O(2). During oxidative stress various other reactive oxygen species are generated that are more reactive than the relatively stable H(2)O(2). Hypochlorous acid (HOCl) is a powerful oxidant which is highly reactive towards a range of biological substrates. We studied the influence of HOCl on the activity of GST P1-1. HOCl inhibits purified glutathione S-transferase P1-1 in a concentration dependent manner with an IC(50)-value of 0.6 microM, which is more than 1000 times as low as IC(50) reported for H(2)O(2). HOCl lowered the V(max) value, but did not affect the K(m) for CDNB. Our results show that HOCl is a potent, non-competitive inhibitor of GST P1-1. The relevance of this effect is discussed.

Gene profiling of cathepsin K deficiency in atherogenesis: profibrotic but lipogenic.

Recently, we showed that cathepsin K deficiency reduces atherosclerotic plaque progression, induces plaque fibrosis, but aggravates macrophage foam cell formation in the ApoE -/- mouse. To obtain more insight into the molecular mechanisms by which cathepsin K disruption evokes the observed phenotypic changes, we used microarray analysis for gene expression profiling of aortic arches of CatK -/-/ApoE -/- and ApoE -/- mice on a mouse oligo microarray. Out of 20 280 reporters, 444 were significantly differentially expressed (p-value of < 0.05, fold change of > or = 1.4 or < or = - 1.4, and intensity value of > 2.5 times background in at least one channel). Ingenuity Pathway Analysis and GenMAPP revealed upregulation of genes involved in lipid uptake, trafficking, and intracellular storage, including caveolin - 1, - 2, - 3 and CD36, and profibrotic genes involved in transforming growth factor beta (TGFbeta) signalling, including TGFbeta2, latent TGFbeta binding protein-1 (LTBP1), and secreted protein, acidic and rich in cysteine (SPARC), in CatK -/-/ApoE -/- mice. Differential gene expression was confirmed at the mRNA and protein levels. In vitro modified low density lipoprotein (LDL) uptake assays, using bone marrow derived macrophages preincubated with caveolae and scavenger receptor inhibitors, confirmed the importance of caveolins and CD36 in increasing modified LDL uptake in the absence of cathepsin K. In conclusion, we suggest that cathepsin K deficiency alters plaque phenotype not only by decreasing proteolytic activity, but also by stimulating TGFbeta signalling. Besides this profibrotic effect, cathepsin K deficiency has a lipogenic effect owing to increased lipid uptake mediated by CD36 and caveolins.

alpha-Tocopherol inhibits human glutathione S-transferase pi.

alpha-Tocopherol is the most important fat-soluble, chain-breaking antioxidant. It is known that interplay between different protective mechanisms occurs. GSTs can catalyze glutathione conjugation with various electrophiles, many of which are toxic. We studied the influence of alpha-tocopherol on the activity of the cytosolic pi isoform of GST. alpha-Tocopherol inhibits glutathione S-transferase pi in a concentration-dependent manner, with an IC(50)-value of 0.5 microM. At alpha-tocopherol additions above 3 microM there was no GST pi activity left. alpha-Tocopherol lowered the V(max) values, but did not affect the K(m) for either CDNB or GSH. This indicates that the GST pi enzyme is noncompetitively inhibited by alpha-tocopherol. An inhibition of GST pi by alpha-tocopherol may have far-reaching implications for the application of vitamin E.