The substance that makes poison ivy poisonous is not an enzyme, but rather an oil called urushiol.

Here's the breakdown:

* Urushiol is a colorless, oily resin found in the sap of poison ivy, poison oak, and poison sumac plants. It is the primary allergen responsible for the itchy, blistering rash that occurs after contact with these plants.

* Not an enzyme: Enzymes are proteins that catalyze (speed up) chemical reactions. Urushiol is not an enzyme; it is a lipid (a type of fat).

* How it works: Urushiol binds to proteins in the skin, triggering an immune response that results in the allergic reaction.

So, while there isn't an enzyme directly responsible for the poison ivy reaction, it's the urushiol oil that causes the trouble.