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  • Title: Novel connections between NADPH-induced lipid peroxidation and cytochrome P450 inactivation, and antioxidant and enzyme protective properties of estradiol in gonadal membranes.
    Author: Kühn-Velten WN, Pippirs U.
    Journal: Free Radic Res; 1997 Feb; 26(2):125-33. PubMed ID: 9257124.
    Abstract:
    This study uses microsomal membranes from rat testis tissue, including the cytochrome P450c17 (steroid 17 alpha-monooxygenase/17 alpha-hydroxyprogesterone aldolase, catalyzing the conversion of progesterone to androstenedione), to decipher the possible relation of NADPH-induced (no exogenous iron added) lipid peroxidation and cytochrome P450 inactivation and the protective effect of certain steroids. NADPH (300 microM) causes a 3.6-fold stimulation of malondialdehyde formation (thiobarbituric acid-reactive substances) and a 29% cytochrome P450c17 loss within 1 h at 37 degrees C, but has no effect on lipid peroxidation in the presence of the iron chelator desferrioxamine. Hydrogen peroxide has only marginal effects. The antioxidant efficiency of estradiol (IC50 = 13.9 microM) is higher than its cytochrome P450c17 protective efficiency (IC50 = 33.0 microM), whereas androstenedione does not inhibit lipid peroxidation but protects cytochrome P450c17 completely. The human choriogonadotropin-induced degradation of cytochrome P450c17 in incubated decapsulated testes can not be correlated with a stimulation of lipid peroxidation, and it is partially inhibited by estradiol but completely abolished by androstenedione. It is concluded (I) that NADPH stimulates iron-dependent generation of reactive oxygen species by the monooxygenase system even in the presence of certain P450 ligands in the physiological membrane environment, (II) that membrane lipid peroxidation may be suppressed by hydrophobic steroids acting as antioxidants such as estradiol, (III) that steroid ligands stabilize cytochrome P450c17 against inactivation in the presence of NADPH even if they do not act as substrates and do not possess antioxidant activity, and (IV) that the choriogonadotropin-induced down-regulation of cytochrome P450c17 is not due to accumulating steroids acting as "pseudosubstrates" as occasionally supposed.
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