This study aims to test if sapropterin (an oral analogue of tetrahydrobiopterin) can play a role in the management of portal hypertension. For this, patients with liver cirrhosis and clinically significant portal hypertension will be randomized to receive sapropterin or placebo for two weeks. Patients will undergo an hepatic vein catheterization to asses the hepatic venous pressure gradient (HVPG), and those having and HVPG of 10 mmHg or higher will be randomized to receive sapropterin or placebo. Swan-Ganz catheterization, systemic measurements, and hepatic blood flow by indocyanine green method will also be performed. Patients will receive sapropterin or placebo for two weeks at a dosage of 5 mg/kg/d in the first week, increasing to 10 mg/kg/d in the second week if there are no adverse events or intolerance. A second systemic and hepatic hemodynamic study will be performed after 2 weeks of treatment to assess changes.