Phenylalanine 4-monooxygenase and the S-oxidation of S-carboxymethyl-L-cysteine by human cytosolic fractions.

The purpose of this investigation was to reaction phenotype the identity of the cytosolic enzyme responsible for the S-oxidation of S-carboxymethyl-L-cysteine (SCMC) in female human hepatic cytosolic fractions. The identity of this enzyme in the female Wistar rat hepatic cytosolic fraction was found to be phenylalanine 4-monooxygenase (PAH). In pooled female human hepatic cytosolic fractions the calculated K(m) and V(max) for substrate (SCMC) activated PAH was 16.22 +/- 11.31 mM and 0.87 +/- 0.41 nmoles x min(-1) mg(-1). The experimental data modelled to the Michaelis-Menten equation with noncompetitive substrate inhibition. When the cytosolic fractions were activated with lysophophatidylcholine the V(max) increased to 52.31 +/- 11.72 nmoles x min(-1) mg(-1) but the K(m) remained unchanged at 16.53 +/- 2.32 mM. A linear correlation was seen in the production of Tyr and SCMC R/S S-oxide in 20 individual female hepatic cytosolic fractions for both substrate and lysophosphatidylcholine activated PAH (r(s) > 0.96). Inhibitor studies found that the specific chemical and antibody inhibitors of PAH reduced the production of Tyr and SCMC R/S S-oxide in these in vitro PAH assays. An investigation of the mechanism of interaction of SCMC with PAH indicated that the drug was a competitive inhibitor of the aromatic C-oxidation of Phe with a calculated K(i) of 17.23 +/- 4.15 mM. The requirement of BH4 as cofactor and the lack of effect of the specific tyrosine hydroxylase, tryptophan hydroxylase and nitric oxide synthase inhibitors on the S-oxidation of SCMC all indicate that PAH was the enzyme responsible for this biotransformation reaction in human hepatic cytosolic fractions.