Properties of NADH-dependent estradiol 2-hydroxylase in rat liver

The NADH-supported cytochrome P-450-dependent 2-hydroxylation of estradiol in rat liver microsomes has been investigated. Estradiol 2-hydroxylation proceeded well with NADH instead of NADPH as a cofactor. Dimethyltetrahydropterine was incapable of serving as a hydrogen donor for this biotransformation. When both NADH and NADPH were used, the 2-hydroxylation increased additively. Molecular oxygen dissolved in the incubation medium was enough for the occurrence of the NADH-dependent 2-hydroxylation. The presence of carbon monooxide suppressed the formation of catechol estrogen where the CO/O2 ratio needed for 50% inhibition of the bioconversion was 7.7. The inhibitory effect was reversed completely by illumination with white light. p-Chloromercuribenzoate inhibited almost completely the 2-hydroxylase activity, and the enzyme activity was also inhibited by SKF-525A. These results strongly imply the possible involvement of a cytochrome P-450 system in the NADH-dependent 2-hydroxylation of estradiol with rat liver microsomes.

Structural modifications in contraceptive steroids altering their metabolism and toxicity

Norethisterone and, to a lesser extent, d-norgestrel are metabolically activated by rat liver microsomal enzymes to intermediates which are capable of irreversibly binding to proteins. This microsomal activation in vitro depends on presence of NADPH and is inhibited by glutathione. Irreversible binding of metabolites of progesterone, nortestosterone acetate and cyproterone acetate is very low, compared to that of norethisterone metabolites. Phenol as a reference compound shows quantitatively a similar binding behaviour as norethisterone. Norethisterone-4beta,5beta-epoxide, a microsomal metabolite of norethisterone, binds non-enzymatically to albumin, at a rate of 380 pmol/mg albumin per hour (at 37 degrees). The corresponding rate for norgestrel-4beta,5beta-epoxide, 42 pmol/mg albumin per hour, indicates a considerably lower reactivity of norgestrel-epoxide. The non-SH-proteins concanavalin A and bovine gamma-globulin do not react with either norethisterone-epoxide or norgestrel-epoxide. Also, DNA and RNA show no binding reaction. Thus, the requirements for irreversible protein binding of the 19-nortestosterone progestagens norethisterone and norgestrel are similar to those found for oestrogens which, when activated by rat liver microsomes, only bind to proteins with SH-groups, not to DNA or RNA.

The significance of covalent binding of catechols to proteins in vivo

When rats were dosed with 14 microgram/kg 3H-isoproterenol, 3H-radioactivity was measurable in the liver until 48 h. This amount was not different in livers of animals which have been pretreated with diethyl maleate. After exhaustive extraction, a significant amount of 3H-radioactivity from isoproterenol could be detected in the proteins of total liver (homogenate), of cytosol and of microsomes. In the cytosol fraction of diethyl maleate pretreated animals twice the amount of isoproterenol-radioactivity was found in the extracted proteins compared to controls. In the microsomal fraction there was no difference between diethyl maleate pretreated and control animals in the amount of radioactivity incorporated into proteins. In all fractions the radioactivity measurable in the extracted proteins declined with a half life time of about 24 h. The in vivo results on covalent binding of isoproterenol are compared to the irreversible protein binding of ethinyloestradiol in vivo. Quantitatively, these in vivo data are compared to the results on irreversible protein binding obtained during incubations of isoproterenol or ethinyloestradiol with rat liver microsomes.