Hydrophobic Aromatic Amino Acid Breath Test

Hydrophobic (Aromatic) Amino Acid Breath Test 

The liver is also the principal metabolic center for hydrophobic amino acids, and hence changes in plasma concentrations or metabolism of these molecules is a good measure of the functional capacity of the liver. Two of the commonly used aromatic amino acids are phenylalanine and tyrosine, which are primarily metabolized by cytosolic enzymes in the liver [1,114-117]. Hydroxylation of phenylalanine to tyrosine by phenylalanine hydroxylase is very efficient by the liver first pass effect. In normal functioning liver, conversion of tyrosine to 4-hy-droxyphenylpyruvate by tyrosine transaminase and subsequent biotransformation to homogentisic acidby 4-hydroxyphenylpyruvic acid dioxygenase liberates CO2 from the C-1 position of the parent amino acid (Fig. 5) [1,118]. Thus, the C-1 position of phenylalanine or tyrosine is typically labeled with and the expired C02 is proportional to the metabolic activity of liver cytosolic enzymes, which corresponds to functional hepatic reserve. Oral or intravenous administration of the amino acids is possible [115]. This method is amenable to the continuous hepatic function measurement approach by monitoring changes in the spectral properties of tyrosine pre- and post-administration of the marker. 

Generally, the liver is the center of drug metabolism, hence numerous drugs and methods to measure functional hepatic capacity are available. A host of these methods rely on the metabolic activity of cytochrome P450 enzymes, and some of the markers used include phenacetin, methacetin, trimethadione. 

Newer and potentially better hepatic function index continues to captivate researchers. A recent report indicates that hippurate ratio is a good measure of functional hepatocyte capacity [13]. The procedure monitors the conjugation of glycine to p-aminobenzoic acid by liver enzymes and may provide unique information on functional hepatic reserve. 

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