Final answer:
Selenium is the mineral used by glutathione peroxidase to function as an antioxidant. It is essential in trace amounts and protects against oxidative stress but can be toxic in excess.
Step-by-step explanation:
Glutathione peroxidase functions as an antioxidant and utilizes the mineral selenium as a cofactor to perform its function. Selenium plays a critical role in the enzyme's ability to catalyze the conversion of reduced glutathione to its oxidized form. It is necessary in trace amounts and is involved in the protection of cellular membranes, prevention of glutathione depletion, free radical scavenging, and it aids in the regeneration of vitamins C and E while protecting DNA, RNA, proteins, and lipids. However, it is important to note that while selenium is essential, excessive amounts can cause selenosis, a toxic effect.