Tyrosine Metabolism in Patients with Liver Disease*

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Abstract
Plasma levels of tyrosine were assayed in the fasting state and after oral administration of either tyrosine (tyrosine tolerance test) or phenylalanine (phenlyalanine conversion test) in normal subjects and in patients with hepatitis, biliary obstruction, or cirrhosis. Fasting tyrosine levels tended to be slightly increased in patients with hepatitis and biliary obstruction and markedly increased in patients with cirrhosis. Tyrosine tolerance tests in patients with cirrhosis were characterized by larger than normal increments in tyrosine levels and by delayed returns toward fasting levels. The results of phenylalanine conversion tests were abnormal in approximately one-half of patients with either hepatitis or biliary obstruction and four-fifths of patients with cirrhosis. Abnormalities were characterized by elevated fasting plasma tyrosine levels, or small and delayed increments in tyrosine levels, or both. Abnormal phenylalanine conversion test results in patients with cirrhosis did not correlate closely with any clinical feature of cirrhosis or with the results of any standard liver function test; there was positive correlation only with abnormal ammonia tolerance, a test of portalsystemic shunting. Tests of tyrosine metabolism do not appear to be useful for routine clinical assessment of liver function. Tyrosine tolerance tests and phenylalanine conversion tests done for purposes of diagnosis of other diseases may yield misleading results in patients with liver disease.