The activity of wild type and mutant phenylalanine hydroxylase with respect to the C-oxidation of phenylalanine and the S-oxidation of S-carboxymethyl-L-cysteine.

[classical phenylketonuria]

The involvement of the enzyme , phenylalanine hydroxylase (PAH ), in the S-oxidation of S-carboxymethyl-L-cysteine (SCMC ) is now firmly established in man and rat . However , the underlying role of the molecular genetics of PAH in dictating and influencing the S-oxidation polymorphism of SCMC metabolism is as yet unknown . In this work we report that the S-oxidation of SCMC was dramatically reduced in the tetrahydrobiopterin (BH (4 )) responsive mutant PAH proteins (I 65 T , R 68 S , R 261 Q , V 388 M and Y 414 C ) with these enzymes possessing between 1 .2 % and 2 .0 % of the wild type PAH activity when SCMC was used as substrate . These same mutant proteins express between 23 % and 76 % of the wild type PAH activity when phenylalanine was used as the substrate . The PAH mutant proteins (R 158 Q , I 174 T and R 408 W ) that result in the classical phenylketonuria (PKU ) phenotype expressing 0 .2 -1 .8 % of the wild type PAH activity when using phenylalanine as substrate were found to have <0 .1 % of the wild type PAH activity when SCMC was used as the substrate . Mutations that result in PAH proteins retaining some residual PAH activity with phenylalanine as substrate have <2 .0 % residual activity when SCMC was used as a substrate . This investigation has led to the hypothesis that the S-oxidation polymorphism in man is a consequence of an individual carrying one mutant PAH allele which has resulted in the loss of the ability of the residual PAH protein to undertake the S-oxidation of SCMC in vivo .