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.
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 (I65T, R68S, R261Q, V388M and Y414C) 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 (R158Q, I174T and R408W) 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.
Steventon
Glyn B
GB
King's College London, Pharmaceutical Science Division, Franklin Wilkins Building, 150 Stamford Street, London SE1 9NH, UK. glyn.steventon@kcl.ac.uk
Mitchell
Stephen C
SC
Pérez
Belen
B
Desviat
Lourdes R
LR
Ugarte
Magdalena
M
D002233:Carbocysteine / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D020125:Mutation, Missense; D010084:Oxidation-Reduction; D010649:Phenylalanine / Q000378:metabolism*; D010651:Phenylalanine Hydroxylase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D010661:Phenylketonurias / Q000235:genetics / Q000378:metabolism*; D013379:Substrate Specificity
D010649:Phenylalanine; D002233:Carbocysteine; D010651:Phenylalanine Hydroxylase