High correlation between results of the [1-13C]-phenylalanine breath test and phenylalanine hydroxylase (EC 1.14.16.1) activity of the liver in rats

Background: (13)CO(2) is decreased in patients with end-stage liver disease by the [1-(13)C]-phenylalanine breath test. Decreased (13)CO(2) is supposed to be caused by the decreased ability of the liver to oxidize phenylalanine. However, no direct evidence has been reported.

Methods: The [1-(13)C]-phenylalanine breath test was performed in galactosamine hepatitis rats (n = 14) and control rats (n = 8). Plasma phenylalanine concentration before intravenous administration of [1-(13)C]-phenylalanine, the elimination rate of phenylalanine and the phenylalanine hydroxylase (PAH; EC 1.14.16.1) activity of the whole liver were examined.

Results: Increase of (13)CO(2) in the breath [Delta(13)CO(2) ( per thousand)] of galactosamine hepatitis rats 2 min after administration of [1-(13)C]-phenylalanine was only 1/5 of that in control rats. The concentration of plasma phenylalanine and the elimination rate of plasma phenylalanine in hepatitis rats did not show significant differences compared to control rats. On the other hand, a clear difference in the activity of PAH was observed between hepatitis rats and control rats. Delta(13)CO(2) ( per thousand) 2 min after administration of [1-(13)C]-phenylalanine was highly correlated to the PAH activity of the whole liver (r = 0.917).

Conclusion: It was strongly indicated that decreased Delta(13)CO(2 ) in hepatitis rats was the result of decreased activity of PAH.