**Question:** Free radicals can be inactivated by the following enzymes EXCEPT

A. Superoxide dismutase (SOD)
B. Catalase
C. Glutathione peroxidase (GPx)
D. Glutathione reductase

**Correct Answer:** D. Glutathione reductase

**Core Concept:**
Free radicals are highly reactive molecules containing unpaired electrons, which can cause damage to cellular components when present in excess. In the body, various enzymes and antioxidant molecules act to neutralize or detoxify these free radicals, protecting cells from oxidative stress-induced injury.

**Why the Correct Answer is Right:**
Glutathione reductase (GR) is an enzyme involved in the redox regulation of glutathione (GSH) system. It catalyzes the reduction of glutathione disulfide (GSSG) back to GSH, using NADPH as the electron donor. However, GR does not play a direct role in inactivating free radicals.

**Why Each Wrong Option is Incorrect:**
A. Superoxide dismutase (SOD) converts superoxide radicals into hydrogen peroxide and oxygen, preventing further damage to cells and other molecules. SOD is crucial in neutralizing superoxide radicals, which are precursors to other free radicals like hydrogen peroxide and hydroxyl radicals.

B. Catalase is an enzyme that catalyzes the conversion of hydrogen peroxide into water and oxygen, thus protecting cells from hydrogen peroxide-induced oxidative stress. Catalase plays a significant role in inactivating hydrogen peroxide, a product of superoxide dismutation by SOD and a precursor of hydroxyl radical formation.

C. Glutathione peroxidase (GPx) is an enzyme that reduces various organic peroxides, including hydrogen peroxide, using glutathione as a reducing agent. GPx directly participates in inactivating hydrogen peroxide and other organic peroxides.

D. Glutathione reductase (GR) is involved in maintaining the reduced state of glutathione (GSH) by catalyzing the reduction of glutathione disulfide (GSSG) to GSH using NADPH as the electron donor. Although GR plays a role in maintaining GSH levels, it does not directly neutralize free radicals or participate in the inactivation of hydrogen peroxide.

**Clinical Pearl:**
Understanding the roles of various antioxidant enzymes helps in appreciating their synergistic actions in protecting cells from oxidative stress. A proper balance of these enzymes is crucial for maintaining cellular redox homeostasis and preventing cellular damage due to excessive free radicals and reactive oxygen species.