Acute coronary syndrome (ACS) is a range of conditions associated with decreased blood flow in the coronary arteries resulting from sudden rupture of atherosclerotic plaques. Several studies have found that oxidative stress is involved in the initiation and progression of atherosclerosis. The role of oxidase enzymes and antioxidative stress biomarkers in these processes needs further attention. In this study, a total of 120 participants were enrolled which comprised 60 ACS patients and 60 control subjects. The major oxidase enzyme, xanthine oxidase, which plays a pivotal role in the generation of reactive oxygen species (ROS), showed significantly higher activities in both serum (158.03 ± 43.30 mU/mL) and red blood cells (RBC) lysate (309.07 ± 75.73 mU/mL) of the ACS patients compared to controls, 48.51 ± 13.41 mU/mL and 184.10 ± 70.14 mU/mL, respectively. The nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and myeloperoxidase, both of which are major contributors to ROS production, showed significantly higher activities in patients (14.13 ± 3.38 U/L and 10.9 ± 3.3 U/mg) than in controls, 6.90 ± 1.94 U/L and 5.9 ± 1.5 U/mg, respectively. Ceruloplasmin, an emerging biomarker of inflammation, showed significantly higher activity in patients (83.8 ± 26.2 U/L) compared to controls, 70.0 ± 18.9 U/L. The antioxidant enzyme glutathione reductase showed significantly lower activity in patients than controls, 60.7 ± 47.8 U/mL/min and 85.2 ± 49.5 U/mL/min, respectively. Evaluation of cardioprotective biomarkers nitric oxide and high-density lipoprotein-cholesterol (HDL-C) showed significantly lower values in patients. Correlation analyses between these parameters further corroborated increased oxidative stress in patients. These findings suggest that excessive productions of ROS by the oxidase enzymes cause an imbalance between How to cite this paper: Ferdausi, N.,