Streptolysin O is inactivated by
**Core Concept**
Streptolysin O is a hemolysin produced by Streptococcus pyogenes, a bacterium that causes strep throat and other infections. This toxin causes cell lysis by forming pores in the cell membrane, leading to leakage of cellular contents. The inactivation of streptolysin O is crucial for preventing cell damage and promoting recovery.
**Why the Correct Answer is Right**
Streptolysin O is inactivated by oxygen, specifically by the presence of oxygen in the environment. This is because oxygen binds to the enzyme, preventing it from causing cell damage. The binding of oxygen to the enzyme causes a conformational change, which inactivates the toxin. This inactivation is a protective mechanism that prevents the toxin from causing further damage.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because streptolysin O is not inactivated by heat. Heat can denature the enzyme, but it is not the primary mechanism of inactivation.
**Option B:** This is incorrect because streptolysin O is not inactivated by pH changes. The enzyme is active over a wide pH range, and changes in pH do not affect its activity.
**Option C:** This is incorrect because streptolysin O is not inactivated by antibiotics. Antibiotics may inhibit the growth of the bacteria that produce the toxin, but they do not directly inactivate the toxin.
**Clinical Pearl / High-Yield Fact**
It is essential to note that streptolysin O is inactivated by oxygen, which is why it is often more toxic in anaerobic environments. This is a key consideration in the treatment of strep throat and other infections caused by Streptococcus pyogenes.
**Correct Answer: D. Oxygen**