Studies on rat liver microsomal steroid metabolism using 18O-labelled testosterone and progesterone

In order to investigate the possible involvement of oxygen functions in the rat liver microsomal metabolism of progesterone and testosterone these steroids were specifically labelled with 18O in their oxo-functions and incubated with NADPH supplemented 105,000 g sediments. Gas chromatography-mass spectrometry was used to identify the metabolites formed as well as to quantitate the losses of 18O-label. With 18O-labelled testosterone as substrate two of the major monohydroxylated metabolites, i.e. 2 beta- and 6 beta-hydroxytestosterone were shown to have lost about 25 and 50% of their 18O respectively. A complete retention of label was found in 7 alpha- and 16 alpha-hydroxytestosterone. None of the monohydroxylated progesterone metabolites, i.e. the 2 alpha-, 6 beta- and 16 alpha-hydroxyprogesterone had lost any 18O following incubation with 3,20-18O-labelled progesterone. Control incubation (30', 37 degrees C) with buffer and 18O-labelled progesterone and testosterone revealed no exchange of 18O. Thus the partial loss of 3-18O-label during 2 beta- and 6 beta-hydroxylation of testosterone may indicate a covalent interaction between the steroid 3-oxo-group and one or more cytochrome P-450 species in the rat liver microsomes. In view of the potentiating effect of a 3-imine group in spontaneous 6 beta-hydroxylation the present in vitro data suggest that a steroid protein-interaction may occur via a 3-imine group during 6 beta-hydroxylation of testosterone in rat liver microsomes. Analysis of 5 alpha-reduced metabolites of both progesterone and testosterone showed significant losses of 3-18O, but due to the ease with which 3-oxo-5 alpha-steroids exchange their 3-18O with aqueous media an enzymatically induced loss of 3-18O could not be safely established. The 20-oxido-reductase which converted progesterone did not induce a loss of 20- or 3-18O thus indicating that the oxofunctions were not covalently engaged in the enzymatic binding of the steroid.