Enalapril has an L-proline moiety as a part of the molecule which is responsible for the oral bioavailability of the drug. It is a pro-drug, which means that it exerts its function after being metabolized. The "-OCH2CH3" part of the molecule will split during the metabolism and at the carbon will be a carboxylate, which then interacts with the Zn+2 site of the ACE enzyme. This structural feature and mechanism of metabolism that must occur before the drug can inhibit the enzyme explains why it has a greater duration of action than another similar drug used for the same indication, Captopril. Duration of effect is dose-related; at recommended doses, antihypertensive and haemodynamic effects have been shown to be maintained for at least 24 hours. Enalapril has a slower onset of action than Captopril but a greater duration of action. However, unlike Captopril, Enalapril does not have a thiol moiety.