Monoamine oxidase expression and activity in human placentae from pre-eclamptic and normotensive pregnancies.

A feature of pre-eclampsia is that circulating levels of maternal serotonin (5-hydroxytryptamine) are elevated and placental monoamine oxidase-A (MAO-A) activity, the major factor in the regulation of serotonin levels in pregnancy, is reduced. It is not known whether this is due to a reduced MAO-A protein content or a reduced catalytic turnover of the serotonin by MAO-A; this question has been addressed in the present work. Term placentae from normotensive and pre-eclamptic women were analysed for MAO-A specific mRNA expression (by semi-quantitative RT-PCR), MAO-A protein (by immunohistochemistry and quantitative ELISA, using a MAO-A specific monoclonal antibody), together with MAO activity (using [(3)H] labelled 5-hydroxytryptamine as substrate). Immunohistochemical analysis of placentae from both normotensive and pre-eclamptic women demonstrated that MAO-A protein is located in the cytoplasm of the placental syncytiotrophoblast layer, consistent with a mitochondrial location; no MAO-A protein was found in the nucleus. No MAO-B protein was detected in this placental layer, despite the presence of MAO-B mRNA. The results indicate that both total protein/g fresh weight and MAO-A protein/g fresh weight were approximately 40 per cent lower in pre-eclamptic than in normotensive placentae, but that there was no statistical difference in the expression of MAO-A mRNA in relation to GAPDH or actin mRNA or in MAO-A protein/mg total protein. However, MAO-A activity/g fresh weight was significantly reduced in pre-eclamptic placentae, in agreement with previous findings. This was found to be due to a 60 per cent reduction (P< 0.05) in the catalytic turnover (activity/molecule) of the enzyme. This study has therefore clearly shown that the expression of placental MAO-A specific mRNA and MAO-A protein are not specifically affected in pre-eclampsia, but that the catalytic efficiency of the expressed MAO-A enzyme in pre-eclamptic placentae is greatly reduced. Copyright 2002 Elsevier Science Ltd.