This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: vitamins, Subject name: Biochemistry
Which of the following is not an antioxidant? 
Select and explain. 
A. SOD, B. Vitamin A, C. Gluthione peroxidase, D. Xanthine oxidase
D: Xanthine oxidase
Answer: D: Xanthine oxidase 
Explanation: Antioxidant Vitamin Vitamin E Vitamin C Beta Carotene selenium Metabolic antioxidants Glutathione: Reduced glutathione (GSH) plays a key role in the biological antioxidant enzyme system others: Ceruloplasmin, Transferrin, Albumin, Bilirubin, Haptoglobin enzymatic antioxidants (superoxide dismutase, catalase) ref satyanarayana 4th ed page 661