ANALYSIS OF PHENYLALANINE-HYDROXYLASE GENOTYPES AND HYPERPHENYLALANINEMIA PHENOTYPES USING L-[1-C-13]PHENYLALANINE OXIDATION RATES IN-VIVO - A PILOT-STUDY

Hyperphenylalaninemia (HPA) resulting from deficient activity of pheny lalanine hydroxylase (PAH) is caused by mutations in the human FAH gen e (McKusick 261600). Herein, we report a noninvasive method to: I) est imate whole-body phenylalanine oxidation in patients with HPA and 2) c ompare effects of mutant genotypes on phenotypes. We used oral L-[1-C- 13]phenylalanine as a substrate and measured (CO2)-C-13, formation in the first hour as an index of phenylalanine oxidation rates in: I) pat ients with PKU (n = 6), variant phenylketonuria (PKU) (n = 7) and non- PKU HPA (a = 4); 2) obligate heterozygotes(n = 18); and 3) controls (n = 8), PAH mutations were identified by PCR, denaturing gradient gel e lectrophoresis, and DNA sequencing. Phenylalanine oxidation rates demo nstrated a gene dosage ent:ct; oxidation in heterozygotes was intermed iate between probands and controls. The three classes of I-IPA had dif ferent mean oxidation rates (PKU < variant PKU < non-PKU HPA). The in vivo phenotype (HPA class or whole-body oxidation rate) did not always correspond to prediction from in vitro expression analysis of the mut ation effect on enzyme activity. The findings indicate that the in viv o metrical trait (phenylalanine oxidation rare) is not a simple equiva lent of phenylalanine hydroxylation activity (unit of protein phenotyp e) and, as expected, is an emergent property under the control of more than the PAH locus.