The association of antioxidant enzyme activity with cardiovascular variables in a bi-ethnic population : the SABPA study

Abstract

Motivation - Hypertension is an escalating problem in our South African population, especially among urban black Africans. The development of hypertension may be facilitated by various factors, including oxidative stress. Oxidative stress can be caused by a decrease in antioxidant capacity or an increase in the production of reactive oxygen species (ROS). Previous studies by our research team support the theory of increased oxidative stress in our black population, but it is unknown whether it is the result of decreased antioxidant enzyme activity, or an increase in ROS production. Aims - We aimed to determine whether black men and women have lower antioxidant enzyme activity than white men and women. We further aimed to determine if any relationships exist between cardiovascular variables, such as blood pressure and carotid intima media thickness (CIMT), with antioxidant enzyme activity such as glutathione reductase (GR) and glutathione peroxidase (GPx) among others. Methods - This study is part of the Sympathetic activity and Ambulatory Blood Pressure in Africans (SABPA) study, which was conducted between February 2008 and May 2009. The SABPA study is a cross-sectional target population study and included 101 black and 101 white male, and 99 black and 108 white female teachers from the Dr Kenneth Kaunda Education District in the North West Province of South Africa. Anthropometric and physical activity measurements were performed according to standardized procedures. Cardiovascular measurements included systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP), as measured with the validated Finometer device. The high resolution SonoSite Micromaxx ultrasound system was used to measure CIMT. Cross-sectional wall area (CSWA) was calculated using the formula CSWA = π(d/2 + CIMT)2 – π(d/2)2. Fasting blood samples were obtained for the analyses of glycated haemoglobin (HbA1c), total cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, y-glutamyl transferase (GGT), and C-reactive protein (CRP). The total cholesterol to high-density lipoprotein cholesterol ratio was calculated. The activity of GPx, GR and superoxide dismutase (SOD) were measured using assay kits (Cayman Chemical Company, Ann Arbor, MI, USA) and a Synergy H4 hybrid microplate reader. Catalase (CAT) activity was measured using a fluorometric OxiSelect catalase activity assay kit (Cell Biolabs Inc., San Diego, CA, USA). Total glutathione (GSH) was measured with the BIOXYTECH GSH/GSSG-412 kit on a Bio-Tek FL600 Microplate Fluorescence Reader. ROS (measured as serum peroxides) was measured using the method described by Hayashi et al. Tumor necrosis factor alpha (TNFα) and cotinine were measured using a high sensitivity ELISA kit. Results - When comparing oxidative stress markers between black and white men it was found that the activity of GR was significantly higher in the black men and women when compared to their white counterparts. Results show that ROS was significantly higher in the black men when compared to white men, and both the black men and the black women had significantly higher total glutathione (GSH) levels than their white counterparts. In black women, GPx activity was significantly lower when compared to the white women. It was found that an independent positive association exists between GR activity and CIMT in black men. Meanwhile in black women an independent negative association exists between GPx activity and SBP, as well as between GPx and MAP. No associations were found in the white participants. Conclusion - The comparison of antioxidant enzyme activity in the black and white men revealed that the black men had significantly higher GR activity, total GSH and ROS. Meanwhile in the women it was found that the black women had significantly higher GR activity and total GSH, but significantly lower GPx activity when compared to their white counterparts. Increased GR activity and GSH levels have been linked to oxidative stress. Up-regulation of GR is suspected in the black men and women in order to combat increased depletion of GSH levels during oxidative stress. GPx enzymes are suspected to be inactivated by ROS, therefore allowing further accumulation of ROS and aggravating the state of oxidative stress in these participants. Thus we suggest that both the black men and black women have higher oxidative stress than the white participants. Analyses of associations between cardiovascular variables and antioxidant enzyme activity in the black and white men showed an independent positive association between CIMT and GR in the black men. This may suggest that an increase in oxidative stress, as indicated by increased GR activity and ROS levels, could promote thickening of the carotid intima media, and eventually stimulate the development of atherosclerosis in these participants. Furthermore, a significant negative association between GPx activity and blood pressure measurements was found in the black women. The suspected oxidative stress in these participants, as indicated by increased GR activity and decreased GPx activity, is suspected to facilitate hypertension development in these participants.