The effects of ascorbate and alpha-tocopherol on the NADPH-dependent lipid peroxidation in human placental mitochondria.

Abstract

The effects of ascorbate and alpha-tocopherol as antioxidants and as co-operative factors against NADPH-dependent lipid peroxidation in human placental mitochondria have been studied. The addition of ascorbate at low concentration (up to 50 microM) to the NADPH-generating system resulted in increasing lipid peroxidation and Fe3+ to Fe2+ reduction. High concentration of ascorbate (150 microM), which produced maximal rate of ascorbate-dependent lipid peroxidation, was found to inhibit almost completely NADPH-dependent lipid peroxidation by maintaining too much iron in its reduced form. Either stimulatory or inhibitory effect of ascorbate on NADPH-dependent lipid peroxidation depends on the appropriate Fe3+/Fe2+ ratio. Alpha-tocopherol caused a decrease of NADPH-dependent lipid peroxidation, inhibiting completely this process at 150 microM concentration. The inhibitory effect of alpha-tocopherol increased rapidly with the increasing ascorbate concentration, almost complete inhibition of NADPH-dependent lipid peroxidation being obtained at 25 microM alpha-tocopherol and 50 microM ascorbate. This strong inhibitory combined effect of alpha-tocopherol and ascorbate was independent of the Fe3+/Fe2+ ratio, as alpha-tocopherol is not able to reduce Fe3+ to Fe2+ under the conditions employed. These findings suggest that antioxidant effects of ascorbate in placental mitochondria are mediated by recycling of alpha-tocopherol rather than by strong reduction of Fe3+ to Fe2+. On the basis of the results obtained, we assume that adequate concentrations of alpha-tocopherol and ascorbate in placental tissue may prevent the release of lipid peroxide from placental mitochondria and therefore could be protective against the development of preeclampsia.