Enalapril (MK-421) belongs to a class of medications called angiotensin converting enzyme inhibitors. Normally angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE). Angiotensin II constricts blood vessels, increasing blood pressure. By inhibiting ACE, Enalapril (MK-421) decreases levels of angiotensin II leading to less vasoconstriction and decreased blood pressure.