When progesterone labeled with one isotope and 17-hydroxyprogesterone labeled with another were incubated together in the presence of an 800 ×g × 10 min supernatant fraction of mouse testis homogenate, the ratio of progesterone label to 17-hydroxyprogesterone label in androstenedione and testosterone was greater than in the original mixture. No evidence of a pathway from progesterone to androstenedione not involving 17-hydroxy progesterone could be found. When the same substrate mixture was incubated with a similar homogenate fraction in the absence of both oxygen and cofactors, very little metabolism occurred. The mitochondrial and microsomal fractions were separated from the incubated mixture and the proportion of the 2 substrates in each was determined. The ratios of the 2 in the particulate fractions resembled those in androstenedione and testosterone under normal incubation conditions rather than the ratio added. Thus, it seems that there is a differential distribution of the 2 steroids into the microsomal particle which contains the 17-hydroxylase and 17,20-lyase. This is not due to differential protein binding in the soluble fraction since the same difference in ratios existed when the microsomes were suspended in 0.25m sucrose. The differential distribution was enough to account for the isotope ratio in the androgens if the microsomal enzymes acted upon the substrate mixture in the microsomes instead of the proportions as added. (Endocrinology85: 402, 1969)
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Matsumoto et al. (1969) studied this question.