Phenylalanine hydroxylase (PAH) normally catalyzes the conversion of phenylalanine to tyrosine. In the absence of functional PAH, phenylalanine accumulates to high levels in the blood and is converted to phenylpyruvate and phenyllactate (Clemens et al. 1990; Langenbeck et al. 1992; Mitchell et al. 2011). The extent of these conversions is modulated by genetic factors distinct from PAH, as siblings with the identical PAH defect can produce different amounts of them (Treacy et al. 1996).
Both L-amino acid oxidase (Boulland et al. 2004) and Kynurenine--oxoglutarate transaminase 3 (Han et al. 2004) can catalyze the conversion of phenylalanine to phenylpyruvate and lactate dehydrogenase can catalyze the conversion of the latter molecule to phenyllactate (Meister 1950), in reactions not annotated here.