Evaluation of C-13-phenylalanine and C-13-tyrosine breath tests for the measurement of hepatocyte functional capacity in patients with liver cirrhosis

Liver disease is associated with an abnormal elevation of the plasma concen trations of the aromatic amino acids phenylalanine and tyrosine. The liver is the main site of aromatic amino acid metabolism, particularly the hydrox ylation of phenylalanine to tyrosine and further tyrosine degradation. In t he present study, we have examined the usefulness of the L-[1-C-13]phenylal anine breath test (C-13-PheBT) and L-[1-C-13]tyrosine breath test (C-13-Tyr BT) for the detection of hepatic damage in patients with liver cirrhosis. F irst, the time courses of (CO2)-C-13 excretion after the administration of L-[1-C-13]phenylalanine and L-[1-C-13] tyrosine were compared. The peak tim es (the time expressed in minutes at which (CO2)-C-13 excretion was maximal ) were 20 min in both breath tests, but C-13-TyrBT gave a higher peak than C-13-PheBT. Next, the parameters of C-13-PheBT and C-13-TyrBT were compared with biochemical liver function test values. These parameters were well co rrelated with several liver blood test values conventionally regarded as me asures of hepatocyte functional reserve. Therefore, C-13-PheBT and C-13-Tyr BT may be useful to assess the degree and progression of hepatic dysfunctio n.