The purpose of this study is to determine whether sapropterin (an oral analogue of tetrahydrobiopterin) could have a role in the treatment of portal hypertension secondary to liver cirrhosis. Sapropterin or placebo will be given for two weeks in patients with liver cirrhosis and clinically significant portal hypertension. Systemic and hepatic hemodynamics studies will be performed at baseline and after the intervention to assess the effect of sapropterin.