The mechanism of the isomerisation of delta 5-3,17-androstenedione by the isomerase (3-oxosteroid delta 4-delta 5-isomerase, EC ) of Pseudomonas testosteroni has been reinvestigated with delta 5-[4-beta-2H]androstenedione as substrate in H2O and delta 5-androstenedione in 2H2O. A precise localisation of the label in delta 4-androstenendione has revealed that the previously reported 4 beta leads to 6 beta deuterium transfer accounts for only a part of the reaction. Along with this process, removal of the 4 alpha proton is also occurring. This has already been observed with mammalian isomerases. Hence the assumed difference in mechanism between the bacterial and mammalian enzymes is very unlikely.
The mechanism of the isomerization of androst-5-ene 3,17-dione by the isomerase of bovine adrenals has been reinvestigated using the methodology previously developed for the study of the bacterial enzyme of Pseudomonas testosteroni. However, owing to the lower activity of the mammalian enzyme, competitive non-enzymic reaction cannot be neglected. It has been shown that even in the absence of spontaneous isomerization, epimerization and exchange of the label on C4 takes place in the buffer. This prevents any quantitative discussion of the course of the reaction. It is however possible to conclude that the mechanism we have proposed for the bacterial enzyme that is, besides the classical 4 beta leads to 6 beta transfer and exchange with the medium, a competitive abstraction of the 4 alpha proton, accounts for the data obtained with the mammalian microsomes.
The fetal adrenal cortex lacks expression of the enzyme early on, thus mineralocorticoids (. aldosterone ) and glucocorticoids (. cortisol ) cannot be synthesized. This is significant because cortisol induces type II pneumocytes of the lungs to synthesize and secrete pulmonary surfactant ; without pulmonary surfactant to reduce the alveolar surface tension , premature neonates may die of neonatal respiratory distress syndrome . If delivery is unavoidable (. because of placental abruption , or pre-eclampsia / HELLP syndrome ), then glucocorticoids (. cortisol) can be administered.