5-ALPHA-DIHYDROPROGESTERONE FORMATION IN HUMAN PLACENTA FROM 5-ALPHA-PREGNAN-3-BETA ALPHA-OL-20-ONES AND 5-PREGNAN-3-BETA-YL-20-ONE SULFATE/

5 alpha-Dihydroprogesterone (5 alpha-DHP) is the immediate precursor o f 5 alpha-pregnan-3 alpha-ol-20-one, a potent anxiolytic/anesthetic ag ent in all vertebrate animals tested, including humans. The levels of 5 alpha-DHP in the plasma of pregnant women are very high; and during the third trimester of pregnancy, the blood production rate of this st eroid may exceed 100 mg/24 h. 5 alpha-DHP in maternal plasma, however, cannot be accounted for totally by the metabolism of maternal plasma progesterone. This study was conducted to evaluate the possibility tha t 5 alpha-DHP is synthesized in placenta from 5 alpha-pregnan-3 alpha/ beta-ol-20-ones delivered to the trophoblast via the fetal umbilical b lood. In incubations of placental minces with radiolabelled 5 alpha-pr egnan-3 alpha/beta-ol-20-ones, there is extensive epimerization and th e intermediate, 5 alpha-DHP, is the major product. In other incubation s, 5 alpha-pregnan-3 beta-ol-20-one-sulfate was hydrolysed and the lib erated 5 alpha-pregnan-3 beta-ol-20-one was converted to 5 alpha-DHP b y homogenates of placental tissue, but 5 alpha-pregnan-3 beta-ol-20-on e-sulfate was not. The oxidation of 5 alpha-pregnan-3 beta-ol-20-ones was concentrated in microsome-enriched preparations of placental tissu e and the apparent K(m)s for 5 alpha-pregnan-3 alpha-ol-20-one and 5 a lpha-pregnan-3 beta-ol-20-one were 3.6 mu M and 78 nM, respectively. T he V(max)s for 5 alpha-DHP formation from 5 alpha-pregnan-3 beta-ol-20 -one and 5 alpha-pregnan-3 beta-ol-20-one were, respectively, 336 pmol /min/mg protein and 9.7 nmol/min/mg protein. These oxidation reactions were supported by both NAD(+) and NADP(+). We suggest that progestero ne, which enters the umbilical circulation from its site of synthesis in the syncytiotrophoblast, is metabolized in the fetus to 5 alpha-pre gnan-3 alpha/beta-ol-ones and to 5 alpha-pregnan-3 alpha-ol-20-one sul fates. These metabolites of progesterone, 5 alpha-pregnan-3 alpha/beta -ol-20-one and 5 alpha-pregnan-3 beta-yl-20-one sulfate, formed in the fetus, serve as plasma-borne substrates for trophoblast formation of 5 alpha-DHP. Because of the hemochorioendothelial nature of human plac entation, 5 alpha-DHP secreted from the trophoblast will preferentiall y enter the maternal compartment, thus constituting a maternal plasma progesterone-independent source of 5 alpha-DHP. (C) 1997 Elsevier Scie nce Ltd. All rights reserved.