Plasma thiols and their determinants in phenylketonuria

Objective: Treatment of phenylketonuria (PKU) patients consists of a phenylalanine-restricted diet supplemented with a tyrosine-, vitamin- and oligoelement-enriched amino-acid mixture. Vitamins and oligoelements may be deficient when compliance with the supplemented special formula is poor. Plasma thiol concentrations (especially homocysteine) depend mainly on B-vitamin intake. Our aim was to evaluate the plasma thiol concentrations (homocysteine, cysteine and glutathione) and their determinants (methionine, cobalamin and folate) in PKU patients under dietary treatment compared with age-matched controls. Design and setting: Cross-sectional study performed in a tertiary care Hospital. Subjects: PKU (42) patients under dietary treatment compared with 42 age-matched controls. Interventions: Plasma total homocysteine, cysteine and glutathione were analyzed by HPLC with fluorescence detection. Plasma phenylalanine and methionine were analyzed by ion exchange chromatography. Serum folate and cobalamin were analyzed by radioimmunoassay procedures. Results: Total homocysteine concentrations were significantly lower in the PKU patients compared with the control group (Students t-test; P < 0.0001). Serum folate and cobalamin were significantly higher in the PKU group (t-Student; P < 0.0001) compared with controls. A significantly negative correlation was observed between total homocysteine and folate (r = -0.378; P = 0.016), and between cobalamin and phenylalanine concentrations (r = -0.367; P = 0.022) in the PKU group. Conclusions: Plasma total homocysteine values are lower in the PKU group than in the controls, probably because of high folate values. High phenylalanine values, an indicator of poor dietary compliance, are negatively associated with cobalamin, which might be deficient in some cases.