Measurement of phenylalanine monooxygenase (PAH) activities

Mammalian phenylalanine monooxygenase (phenylalaninase, phenylalanine hydroxylase, PAH; EC 1.14.16.1) is a member of the large aromatic amino acid hydrolase cohort of enzymes that include tyrosine monooxygenase and tryptophane monooxygenase. PAH is a non-heme-iron-dependent protein that normally catalyzes the C-oxidation of phenylalanine (Phe) to tyrosine (Tyr) in the presence of BH(4), utilizing molecular dioxygen as an additional substrate. However, over recent years, the presumed narrow substrate specificity of PAH has been questioned and catalytic activity towards alternative xenobiotic substrates (both environmental and drugs) has been reported. Like the cytochrome P450 system, PAH is able to oxidize both aliphatic and aromatic carbon centers in addition to undertaking the S-oxidation of aliphatic thioethers (including the two mucoactive drugs S-carboxymethyl-L-cysteine and S-methyl-L-cysteine).