**Question:** Respiratory burst is due to which enzyme -

A. NADPH oxidase
B. Myeloperoxidase
C. Cytochrome P-450
D. Lactate dehydrogenase

**Core Concept:**

The respiratory burst is a crucial component of the immune system, particularly in white blood cells (WBCs) like neutrophils and monocytes. This process involves the production of reactive oxygen species (ROS), such as superoxide anions (O₂⁻), hydrogen peroxide (H₂O₂), and hydroxyl radicals (OH⁻). These ROS serve as antimicrobial agents to neutralize pathogens and protect the host from infections. The respiratory burst is facilitated by a group of enzymes, collectively known as NADPH oxidase, which are responsible for the conversion of molecular oxygen (O₂) into ROS.

**Why the Correct Answer is Right:**

The respiratory burst is primarily attributed to the enzyme NADPH oxidase, which is a membrane-bound enzyme complex found within the phagolysosome of phagocytic cells. The enzyme is composed of five subunits: p22phox, p47phox, p40phox, p67phox, and Rac2. These subunits combine to form NADPH oxidase, which facilitates the conversion of molecular oxygen (O₂) into superoxide anion (O₂⁻) and other ROS.

**Why Each Wrong Option is Incorrect:**

**A.** Myeloperoxidase (MPO) is an enzyme primarily found in neutrophils and is involved in the production of hypochlorous acid (HOCl) from hydrogen peroxide (H₂O₂) and chloride ions (Cl⁻). MPO is distinct from NADPH oxidase and is responsible for antimicrobial activity within the phagolysosome. It is not involved in the respiratory burst.

**B.** Cytochrome P-450 is a family of heme enzymes found in the endoplasmic reticulum of hepatocytes and other cells. It is involved in the synthesis of cholesterol, lipids, and steroids. Cytochrome P-450 is unrelated to the respiratory burst process in phagocytic cells.

**C.** Lactate dehydrogenase (LDH) is an enzyme found in the cytoplasm of various cells, primarily involved in the conversion of lactate to pyruvate in the citric acid cycle. LDH does not play a role in the respiratory burst process in phagocytic cells.

**D.** Ribonucleotide reductase is an enzyme involved in DNA synthesis and repair in cells. It is not associated with the respiratory burst process in phagocytic cells.

**Clinical Pearl:**

The respiratory burst is a critical component of the immune response involving a coordinated interaction between various enzymes, including NADPH oxidase, myeloperoxidase, and lysozyme. This process helps eliminate pathogens and generate antimicrobial agents within phagocytic cells, such as neutrophils and macrophages. Understanding respiratory burst is essential for medical students preparing for exams like NEET PG,