Enzyme that protects the brain from free radical injury is:
Free radicals, like superoxide anions, are produced during normal metabolism and can cause cellular damage. The brain is particularly vulnerable because it has a high oxygen consumption and is rich in polyunsaturated fatty acids, making it susceptible to lipid peroxidation. So the enzyme in question must be one that neutralizes these radicals.
Superoxide dismutase is the first line of defense. It converts superoxide radicals into hydrogen peroxide and oxygen. Then, catalase and glutathione peroxidase break down hydrogen peroxide into water and oxygen. However, in the context of the brain, which is a high-metabolism organ, SOD is especially important because it's the first step in this process. Without SOD, hydrogen peroxide would accumulate, leading to more damage.
Now, the options were A to D, but the correct answer is likely SOD. The other enzymes, like catalase, are present but not the primary one. Glutathione peroxidase is another antioxidant enzyme but uses glutathione as a substrate, not as an enzyme itself. Also, there's catalase, which is more in peroxisomes. Maybe the question is testing knowledge of primary antioxidants in the brain.
Wait, the options weren't provided, but the correct answer is SOD. So the explanation should focus on SOD's role. Let me make sure there's no other enzyme. For example, some might confuse it with catalase, but catalase is more for hydrogen peroxide breakdown. SOD is the first step. Also, in the brain, SOD is present in different isoforms, like Cu/Zn-SOD in the cytoplasm and Mn-SOD in mitochondria. That's important because mitochondria are a major source of ROS.
The incorrect options might include enzymes like acetylcholinesterase, which is unrelated to free radicals. Or maybe catalase, which is a later step. So the key is that SOD is the primary enzyme that converts superoxide radicals, the main free radicals in the brain.
**Core Concept**
The brain is highly susceptible to oxidative stress due to its high oxygen consumption and lipid content. Antioxidant enzymes like **superoxide dismutase (SOD)** neutralize reactive oxygen species (ROS), preventing cellular damage. SOD catalyzes the dismutation of superoxide radicals (O₂⁻) into hydrogen peroxide (H₂O₂), which is further detoxified by catalase and glutathione peroxidase.
**Why the Correct Answer is Right**
**Superoxide dismutase (SOD)** is the primary enzyme protecting the brain from free radical injury. It converts superoxide radicals (O₂⁻), generated during mitochondrial respiration and other metabolic processes, into hydrogen peroxide (H₂O₂) and oxygen (O₂). This prevents the formation of more reactive species like hydroxyl radicals (•OH) via the Haber-Weiss reaction. SOD exists in isoforms (e.g., Cu/Zn-S