Abstract
Objectives The aim of this investigation was to provide in-vitro enzyme kinetic data to support the hypothesis that the in-vivo heterozygous dominant phenotype for phenylalanine monooxygenase (hPAH) was responsible for the S-oxidation polymorphism in the metabolism of S-carboxymethyl-l-cysteine reported in humans. Using a dual-vector expression strategy for the co-production of wild-type and mutant human hPAH subunits we report for the first time the kinetic parameters (K(m) , V(max) , CL(E) ) for the C-oxidation of l-phenylalanine and the S-oxidation of S-carboxymethyl-l-cysteine in homomeric wild-type, heteromeric mutant and homomeric mutant hPAH proteins in vitro. Methods A PRO(TM) dual-vector bacterial expression system was used to produce the required hPAH proteins. Enzyme activity was determined by HPLC with fluorescence detection. Key findings The heteromeric hPAH proteins (I65T, R68S, R158Q, I174T, R261Q, V338M, R408W and Y414C) all showed significantly decreased V(max) and CL(E) values when compared to the homomeric wild-type hPAH enzyme. For both substrates, all calculated K(m) values were significantly higher than homomeric wild-type hPAH enzyme, with the exception of I65T, R68S and Y414C heteromeric hPAH proteins employing l-phenylalanine as substrate. Conclusions The net outcome for the heteromeric mutant hPAH proteins was a decrease significantly more dramatic for S-carboxymethyl-l-cysteine S-oxidation (1.0-18.8% of homomeric wild-type hPAH activity) when compared to l-phenylalanine C-oxidation (25.9-52.9% of homomeric wild-type hPAH activity) as a substrate. Heteromeric hPAH enzyme may be related to the variation in S-carboxymethyl-l-cysteine S-oxidation capacity observed in humans.