Properties of enzyme systems involved in the formation of catechol estrogen glutathione conjugates in rat liver microsomes

Abstract

The properties of enzyme systems involved in the formation of the glutathione 1- and 4-thioethers of catechol estrogen from estradiol, 2-hydroxy-3-deoxyestradiol, or 2-hydroxyestradiol in rat liver microsomes have been investigated. Molecular oxygen was essential and NADPH was preferable as a cofactor to obtain the maximum activity with the three substrates. The presence of carbon monoxide suppressed the formation of the thioethers where the CO O 2 ratios needed for 50 per cent inhibition of the bioconversion were 2.1 to 3.6. This inhibitory effect was reversed almost completely by illumination with white light. SKF-525A inhibited considerably the formation of the glutathione conjugates. Pretreatment with phenobarbital stimulated the formation of the thioethers from the three substrates by 100–220 per cent, whereas administration of 3-methylcholanthrene did not exert any significant influence. The storage of frozen microsomes resulted in a marked decrease in the enzyme activity: the initial activity was depressed to 50 per cent at 24 hr for catechol estrogen and at 4–5 days for phenol estrogens. The NADH-dependent enzyme activities were also inhibited by both SKF-525A and CO; the CO inhibition was reversed by light irradiation. It is evident from these data that cytochrome P-450 participated in both NADPH- and NADH-dependent formation of 2-hydroxyestradiol glutathione thioethers and two different enzyme systems are involved in this biotransformation between phenol estrogens and catechol estrogen.