J. Inher. Metab. Dis. 21 236 È239 (1998) SSIEM and Kluwer Academic Publishers. Printed in the Netherlands ( Short Communication Phenylalanine and tyrosine metabolism in phenylketonuria heterozygotes : Influence of different phenylalanine hydroxylase mutations M. SPADA1*, I. DIANZANI1, G. BONETTI2, A. BIONDI2, L. LEONE2, S. GIANNATTASIO3 and A. PONZONE1,4 of Pediatrics and of of 1 Departments Genetics, University T orino ; 2 Department Clinical Margherita ChildrenÏs Chemistry, Regina Hospital, T orino ; 3 CNR-Centro di Studio sui Mitocondri e Metabolismo of Formation Energetico, Bari ; 4 Faculty of Sciences, University Messina, Italy * Correspondence : Clinica Pediatrica, Piazza Polonia, 94, I-10126 T orino, Italy di†erent tests available to identify phenylketonuria (PKU ; McKusick 261600) The heterozygotes have been excellently summarized by and and Lehman (1989) Scriver colleagues When practical methods were employed, all studies showed that (1995). PKU carriers di†ered as a group from noncarriers in their phenylalanine (Phe) metabolizing capacity. However, no test was devoid of a certain error rate due to overlap between the two groups. A single study has investigated whether the sever- ity of a mutation in the phenylalanine hydroxylase (PAH) gene is reÑected in fasting serum Phe and tyrosine (Tyr) concentrations of PKU heterozygotes et al (Svensson 1994). This study investigated whether mutant PAH alleles inÑuence Phe and Tyr metabolism in PKU heterozygotes after oral Phe loading according to their severity. MATERIALS AND METHODS DNA was extracted from peripheral blood of parents and siblings of PKU patients whose phenotype/genotype correlations have previously been described et (Dianzani al PAH exons were ampliÐed and mutation detection was performed by 1995). restriction enzyme analysis using an enzyme appropriate for each mutation identi- Ðed in the proband. Five heterozygotes carried severe mutations (S67P, L197fsde122, IVS10nt546, R261X, IVS7nt1) ; Ðve carried mild mutations (two the R158Q, two the L48S, one the R261Q) ; and three carried benign or non-PKU mutations (two the A403V, one the E390G), respectively. The severity of mutations had been assessed either clinically, in homozygous patients, or by in vitro assay of 236