Distribution of progesterone-4-14C and 17-hydroxyprogesterone-7-3H between microsomes and sucrose-buffer when equilibrium was achieved without metabolism, and as precursors when metabolism to androstenedione occurred, indicated that the ratio of the two compounds in the rnicrosomes rather than in the medium determined the proportion of the two precursors in the C19 steroids. The isotope ratios in microsomal and supernatant 17-hydroxyprogesterone indicated that the active sites of 17α-hydroxylase and 17,20- lyase were so closely associated that 17-hydroxyprogesterone or a 17-oxy free radical intermediate formed from progesterone did not equilibrate with added 17-hydroxyprogesterone in the microsomes before being converted to androstenedione. No such relationship existed between 17,20-lyase and 17β(testosterone)-dehydrogenase. Relative concentrations of the different compounds, as distinguished from precursor ratios, indicated that 17-hydroxyprogesterone, androstenedione and testosterone distributed like 3H2O, and therefore probably diffused relatively freely between microsomes and medium, while progesterone was differentially concentrated in the microsomes. (Endocrinology94: 55, 1974)
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Samuels et al. (1974) studied this question.