Original articleIncreased biological oxidation and reduced anti-oxidant enzyme activity in pre-eclamptic placentae
Introduction
Placental oxidative stress has been shown to be a key feature in the pathogenesis of pre-eclampsia (PET) (for reviews see [1], [2], [3]). Oxidative stress is defined as an imbalance between the cellular generation of reactive oxygen species (ROS) and the capacity of anti-oxidants to prevent oxidative damage. Enzymatic processes generate ROS, largely by oxidases, or by electron leakage from the mitochondrial electron transport chain. The addition of 1 electron to molecular oxygen results in the production of superoxide (O2−∗), which may be metabolised by the superoxide dismutase family of enzymes to form hydrogen peroxide (H2O2) or react with nitric oxide (NO) to form peroxynitrite (OONO). Peroxynitrite is potentially harmful as it causes nitrosylation of tyrosine residues leading to changes in protein conformation and inactivation. Hydrogen peroxide is able to diffuse through cellular membranes where, in the case of the mitochondrial membrane, it abstracts an electron from Fe2+ ions to generate the highly reactive hydroxyl radical (OH∗). The hydroxyl radical can cause lipid peroxidation, compromising mitochondrial ATP production and leading to loss of cytochrome c, a pro-apoptotic event [4].
Reactive oxygen species are sequestered by anti-oxidants that may be non-protein based, such as vitamins E, C and A or metabolites such as glutathione, ubiquinone and uric acid. Protein-based anti-oxidant enzymes include Mn or Cu superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx) and thioredoxin peroxidase (TPx). The latter two are fundamentally important in maintaining mitochondrial homeostasis as both are located within the mitochondria and are able to inactivate H2O2 produced by Mn-SOD within the mitochondrial matrix before it gains access to the inner mitochondrial membrane. When GPx reacts with H2O2 it becomes oxidised and must be reduced by electron shuffling from glutathione and glutathione reductase in order to maintain activity. Likewise, the TPx enzyme becomes oxidised when reacting with H2O2 and requires thioredoxin (Thx) and thioredoxin reductase (ThRed) to be regenerated. In both cases NADPH is the electron donor for this reductive recycling of enzyme activity as these thiol-based enzymes are inactive when in an oxidised state [5].
Several reports have examined the by-products of excessive cellular oxidation in pre-eclamptic placentae including the measurement of lipid hydroperoxides [6], protein carbonyls [7], and nitrotyrosine residues [8]. The vast majority of these studies have shown increased levels of lipid and protein oxidation in pre-eclamptic placentae and the consensus of opinion amongst researchers in this field is that there is excessive production of ROS in the placentae of women experiencing pre-eclampsia. However, less is known on the endogenous production of key anti-oxidant proteins in the human placenta during pre-eclampsia. Therefore, the purpose of this study was to examine the placental oxidative state and the activity of key anti-oxidant enzymes in tissue homogenates of placental samples taken from age-matched pre-eclamptic and normal pregnancies.
Section snippets
Collection of placental samples
Pregnant women were recruited during antenatal visits to the John Hunter Hospital, Newcastle, and the Royal Women's Hospital, Melbourne. The Hunter Health Research Ethics Committee, the University of Newcastle Human Ethics committee and the Royal Women's Hospital Human Ethics Committee granted ethics approval for the collection and study of human placental tissues. Pre-eclamptic patients were selected for study after demonstrating symptoms of pre-eclampsia as defined by the Council of the
Results
Placental tissue samples from 18 normal and 20 pre-eclamptic patients matched for gestational age were analysed to assess placental oxidative state and compare the levels of superoxide dismutase, glutathione peroxidase, thioredoxin reductase and thioredoxin. The oxidative state of pre-eclamptic and normal placental tissues were measured via lipid peroxide and protein carbonyl concentrations. Figure 1A illustrates the significantly increased levels of 4-HNE + MDA found in pre-eclamptic placenta
Discussion
Despite its prevalence and severity, the pathophysiology of pre-eclampsia is still not completely understood. It would appear that maternal endothelial dysfunction and a hypersensitive reaction to placental debris are central to the changes occurring within the mother, and this has been attributed to increased apoptosis in the placenta. Several reports have indicated that there is increased apoptosis and shedding of placental fragments in to the maternal circulation during pre-eclamptic
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