Evaluation of ¹³C-Phenylalanine and ¹³C-Tyrosine Breath Tests for the Measurement of Hepatocyte Functional Capacity in Patients with Liver Cirrhosis

Abstract

Liver disease is associated with an abnormal elevation of the plasma concentrations of the aromatic amino acids phenylalanine and tyrosine. The liver is the main site of aromatic amino acid metabolism, particularly the hydroxylation of phenylalanine to tyrosine and further tyrosine degradation. In the present study, we have examined the usefulness of the L-[1-¹³C]phenylalanine breath test (¹³C-PheBT) and L-[1-¹³C]tyrosine breath test (¹³C-TyrBT) for the detection of hepatic damage in patients with liver cirrhosis. First, the time courses of ¹³CO₂ excretion after the administration of L-[1-¹³C]phenylalanine and L-[1-¹³C]tyrosine were compared. The peak times (the time expressed in minutes at which ¹³CO₂ excretion was maximal) were 20 min in both breath tests, but ¹³C-TyrBT gave a higher peak than ¹³C-PheBT. Next, the parameters of ¹³C-PheBT and ¹³C-TyrBT were compared with biochemical liver function test values. These parameters were well correlated with several liver blood test values conventionally regarded as measures of hepatocyte functional reserve. Therefore, ¹³C-PheBT and ¹³C-TyrBT may be useful to assess the degree and progression of hepatic dysfunction.