Tyrosine metabolism in cirrhosis.

The aim of this study was to define the enzyme defect responsible for tyrosinemia in cirrhotic patients. The principal hepatic degradation pathway for tyrosine, tyrosine leads to p-hydroxyphenylpyruvic acid equilibrium homogentisic acid leads to CO2 was studied in 18 cirrhotic patients and eight controls. The classic method employed in elucidation of hereditary tyrosinosis was sued. Metabolic intermediates on the pathway were measured in the basal state, and following oral loading doses (50 mg/kg BW) of tyrosine, PHPA, and homogentisic acid. Cirrhotic patients showed a significant increase (p = 0.005) in fasting plasma tyrosine and in basal PHPA excretion and impaired tolerance to all three metabolites when compared to normals. Fifteen of the 18 cirrhotic patients showed tyrosine intolerance which was not accompanied by change in distal metabolites compared to their basal levels. Nevertheless 13 of the 18 did exhibit intolerance of either PHPA or homogentisic acid. We conclude that in contrast to the single complete defect in hereditary disorders of tyrosine metabolism, cirrhotic patients have partial defects at tyrosine transaminase, PHPA oxidase, and homogentisic acid oxidase, the initial step being rate-limiting.