The analysis of pesticides and fungicides in the inhibition of human and rat placental 3l3-hydroxysteroid dehydrogenase activity: Mode of inhibition and mechanism

3l3-Hydroxysteroid dehydrogenase/steroid A5,4-isomerase 1 (3l3-HSD1) plays a critical role in the biosynthesis of progesterone from pregnenolone in the human placenta to maintain normal pregnancy. Whether they inhibit placental 3l3-HSD1 and mode of inhibition remains unclear. In this study, we screened 21 pesticides and fungicides in five classes to inhibit human 3l3-HSD1 and compared them to rat homolog 3l3-HSD4. 3l3-HSD activity was measured by catalyzing pregnenolone to progesterone in the presence of NAD+. Of the 21 chemicals, azoles (difenoconazole), thiocarbamates (thiram and ferbam) and organochlorine (hexachlorophene) significantly inhibited human 3l3-HSD1 with half maximal inhibitory concentration (IC50) values of 2.77, 0.24, 0.68, and 17.96 mu M, respectively. We also found that difenoconazole, ferbam and hexachlorophene are mixed/competitive inhibitors of 3l3-HSD1 while thiram is a mixed/noncompetitive inhibitor. Docking analysis showed that difenoconazole and hexachlorophene bound steroid-binding site. Difenoconazole and hexachlorophene except thiram and ferbam also significantly inhibited rat 3l3-HSD4 activity with IC50 of 1.12 and 2.28 mu M, respectively. Thiram and ferbam significantly inhibited human 3l3-HSD1 possibly by interfering with cysteine residues, while they had no effects on rat 3l3-HSD4. In conclusion, some pesticides potently inhibit placental 3l3-HSD, leading to the reduction of progesterone formation.