| Summary: | Phenylketonuria (PKU) and related forms of non-PKU hyperphenylalaninemias (HPA) result from deficiencies in phenylalanine hydroxylase (PAH), the hepatic enzyme that catalyses the conversion of phenylalanine (phe) to tyrosine (tyr). Patients are characterised by a metabolic phenotype comprising elevated levels of phe and some of its metabolites, notably phenyllactate (PLA), phenylacetate (PAA) and phenylpyruvate (PPA), in both tissue and body fluids. Treatment from birth with low-phe diet largely prevents the severe mental retardation that is its major consequence. === Mechanisms underlying the pathophysiology of PKU are still not fully understood; to this end, the availability of an orthologous animal model is relevant. A number of N-ethyl-N-nitrosourea (ENU) mutagenized mouse strains have become available. I report a new heteorallelic strain, developed by crossing female ENU1 (with mild non-PKU HPA) with a male ENU2/+ carrier of a 'severe' PKU-causing allele. I describe the new hybrid ENU1/2 strain and compare it with control (BTBR/Pas), ENU1, ENU2 and the heterozygous counterparts. The ENU1, ENU1/2 and ENU2 strains display mild, moderate and severe phenotypes, respectively, relative to the control and heterozygous counterparts. === I describe a novel method using negative ion chemical ionization gas chromatography/mass spectrometry (NICI-GC/MS) to measure the concentration of PLA, PAA and PPA in the brain of normal and mutant mice. Although elevated moderately in HPA and more so in PKU mice, concentrations of these metabolites are not sufficient to explain impaired brain function; however phe is present in brain at levels associated with harm. === Finally, I describe a new modality for treatment of HPA, compatible with better human compliance: it involves enzyme substitution with non-absorbable and protected phenylalanine ammonia lyase (PAL) in the intestinal lumen, to convert L-phenylalanine to the harmless metabolites (trans-cinnamic acid and trace ammonia). PAL, taken orally, substitutes for the deficient PAH enzyme and depletes body pools of excess phe. I describe an efficient recombinant approach to produce PAL enzyme. I also provide proofs of both pharmacologic and physiologic principles by testing PAL in the orthologous mutant mouse strains with HPA. The findings encourage further development of PAL for oral use as an ancillary treatment of human PKU.
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