Question

Heparin exerts its anticoagulant effect by enhancing the activity of which of the following endogenous inhibitors of coagulation?

• A. Antithrombin III
• B. Protein C
• C. Protein S
• D. Calcium

Hint:

Heparin is one of the oldest and most widely used parenteral anticoagulants. It does not directly dissolve clots; instead it works indirectly by boosting the body's own natural anticoagulant proteins.

Answer 1

The Correct Option is A

Step 1: What does Heparin do?
Heparin is a naturally occurring anticoagulant used to prevent and treat blood clots. It works indirectly by enhancing an endogenous inhibitor of coagulation.

Step 2: The key molecule - Antithrombin III.
Antithrombin III (AT III) is a serine protease inhibitor (serpin) that naturally inactivates several clotting factors, especially thrombin (IIa) and Factor Xa. Without heparin, this process is slow.

Step 3: How Heparin speeds things up.
Heparin binds to Antithrombin III and causes a conformational change. This change greatly accelerates AT III's ability to inhibit thrombin and Factor Xa - by up to 1000 times. This is the main mechanism of heparin's anticoagulant action.

Step 4: Why not Protein C or S?
Protein C and Protein S are anticoagulant proteins that are activated by the thrombomodulin-thrombin complex. Heparin does not enhance their activity. Warfarin, however, reduces Protein C and S levels.

Step 5: Why not Calcium?
Calcium is a procoagulant cofactor. It is removed in some in vitro anticoagulation methods (like EDTA tubes), but heparin does not act by removing or affecting calcium.

Step 6: Conclusion.
Heparin works by enhancing Antithrombin III activity.
Answer: Option (1) — Antithrombin III