Tyrosine transaminase activity in normal and cirrhotic liver

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Abstract
Most cirrhotics have tyrosinemia and subnormal tyrosine tolerance; in some the ability to metabolizep-hydroxyphenylpyruvic and homogentisic acids is impaired. In previous studies, the initial transamination appeared to be the rate-limiting step. In this study, hepatic tyrosine transaminase activity was compared in liver biopsies from eight noncirrhotic and ten cirrhotic subjects to determine whether the subnormal tyrosine tolerance was related to decreased maximal activity of this enzyme. Fasting plasma tyrosine in the cirrhotics (133±43 μmol/liter) was significantly higher (P<0.005) than in the noncirrhotic subjects (64±25 μmol/liter). Tyrosine transaminase activity in the cirrhotic livers (42±11 μmol PHPA/g liver/hr, or 0.47±0.1 μmol PHPA/mg protein/hr) was not significantly different from the enzyme activity in the noncirrhotic liver (43±7 μmol PHPA/g liver/hr, or 0.39±.12 μmol PHPA/mg protein/hr.) Thus elevated tyrosine levels in cirrhotics cannot be explained by decreased tyrosine transaminase activity in the liver, and other explanations must be sought.