Assessment of antioxidative and selenium status by seleno‐dependent glutathione peroxidase activity in different blood fractions using a pig model: issues for clinical nutrition and research

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Glutathione peroxidases (GPXs) are recognized as antioxidants, catalysing the detoxification of hydrogen peroxide and organic hydroperoxides and contributing to the control of reactive oxygen species (ROS) levels. Hydrogen peroxide acts as a physiological signalling molecule (Murphy et al., 2011), and its catalytic reaction by GPX is believed to equally regulate signalling cascades (Brigelius‐Flohé and Flohé, 2011) affecting a myriad of metabolic and physiological functions.
Because most of GPXs (GPX1, GPX2, GPX3 and GPX4) contain selenocysteine in their catalytic site, the activity of these enzymes is dependent on the availability of selenium (Se) (Brigelius‐Flohé and Maiorino, 2013). Therefore, the measurement of SeGPX activity in blood is extensively used as an indicator of Se status (Han et al., 2011; Bermingham et al., 2014), antioxidative status and potential (Dalto et al., 2015b; Ibrahim et al., 2015), as well as a parameter to evaluate the response to therapeutic treatment (Méplan, 2015; Speckmann and Grune, 2015), amongst others.
In animal science research (Acda and Chae, 2002; Mahan and Peters, 2004; Svoboda et al., 2008), as well as in animal (Schoonheere et al., 2009; Todd et al., 2012) and human clinical nutrition (Daniels et al., 2008; Thomson et al., 2008; Xia et al., 2010), responses of GPX activity to dietary Se supplementation are inconsistent. In fact, although GPX activity is a useful biomarker of Se status of an individual, significant heterogeneity is observed between studies (Ashton et al., 2009). A crucial analytical aspect of this heterogeneity might be due to the fact that GPX activity is reported using different reference units (e.g. mg of blood plasma protein, mg of haemoglobin, ml of whole blood and ml of blood plasma) in the literature. According to Esworthy et al. (1999), sample‐quantifying methods such as protein or DNA content are necessary to standardize the analytical data used to express specific GPX enzymatic activity. Protein content is the most commonly used convention, but Esworthy et al. (1999) reported that enzyme activity per unit of volume may be adequate for assessment in fractions of biological samples like plasma, milk or aqueous humour.
In previous studies on Se metabolism carried out in this laboratory, using gilts during the ovulation period as a model of oxidative stress (Fortier et al., 2012; Dalto et al., 2015a,b, 2016), variations in SeGPX activity were observed according to dietary Se. Reference units by which SeGPX activity was expressed [milliunit (mU)/mg haemoglobin and mU/ml whole blood in Dalto et al. (2015a,b) vs. mU/mg blood plasma protein and mU/ml blood plasma in Dalto et al. (2016)] differed between those studies and possibly contributed to discrepancies in terms of results and interpretations.
Using a data set generated from Dalto et al. (2015a, 2016), the present study compares long‐term and peri‐oestrus profiles for SeGPX activities, expressed with different reference units, aiming to explain their dissimilarities and impact on SeGPX activity interpretation.
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