Which enzyme is inhibited by aminophylline?
**Core Concept**
Aminophylline is a medication that belongs to the class of methylxanthines. It is used to treat asthma and chronic obstructive pulmonary disease (COPD) by relaxing the bronchial muscles and improving breathing. The mechanism of action of aminophylline involves the inhibition of phosphodiesterase enzymes, which are involved in the regulation of intracellular levels of cyclic adenosine monophosphate (cAMP).
**Why the Correct Answer is Right**
Aminophylline works by inhibiting the enzyme phosphodiesterase-3 (PDE3), which breaks down cAMP in smooth muscle cells. By inhibiting PDE3, aminophylline increases the levels of cAMP, leading to relaxation of bronchial muscles and improved airflow. This mechanism is similar to that of other methylxanthines, such as theophylline.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because aminophylline does not inhibit cyclooxygenase (COX) enzymes, which are involved in the production of prostaglandins and are targeted by nonsteroidal anti-inflammatory drugs (NSAIDs).
* **Option B:** This option is incorrect because aminophylline does not inhibit angiotensin-converting enzyme (ACE), which is involved in the renin-angiotensin-aldosterone system and is targeted by ACE inhibitors.
* **Option C:** This option is incorrect because aminophylline does not inhibit lipoxygenase, which is involved in the metabolism of arachidonic acid and is targeted by leukotriene modifiers.
**Clinical Pearl / High-Yield Fact**
Aminophylline is often used in combination with beta-2 agonists, such as salbutamol, to treat acute asthma exacerbations. However, it is essential to monitor serum theophylline levels carefully, as toxicity can occur with high doses or in patients with liver disease.
**Correct Answer: C. Phosphodiesterase-3 (PDE3)**