Phenylalanine hydroxylase: Possible involvement in the S-oxidation of S-carboxymethyl-L-cysteine

Activated phenylalanine 4-monooxygenase, phenylalanine hydroxylase (PAH), is known to be involved in the S-oxidation of a number of sulfide compounds. One of these compounds, S-carboxymethyl-l-cysteine (SCMC), is currently used for the treatment of chronic obstructive pulmonary disease and otitis media with effusion as a mucolytic agent, and the S-oxides are the major metabolites found in urine. However, the enzyme catalyzing the S-oxidation of SCMC has yet to be identified. Here we report on the role of nonactivated phenylalanine 4-monooxygenase activity in rat liver cytosol in the S-oxidation of SCMC. Linearity of the enzyme assays was seen for both time (0-16 min) and cytosolic protein concentration (0.1-0.5 mg/ml). The calculated Km and V max values for the formation of SCMC (S) S-oxide were 3.92 +/- 0.15 mM and 1.10 +/- 0.12 nmol SCMC (S) S-oxide formed/mg protein/min, respectively. The calculated Km and Vmax values for the formation of SCMC (R) S-oxide were 9.18 +/- 1.13 mM and 0.46 +/- 0.11 nmol SCMC (R) S-oxide formed/mg protein/min, respectively. These results indicate that in the female Wistar rat, nonactivated PAH showed a stereospecific preference for the formation of the (S) S-oxide metabolite of SCMC against the (R) S-oxide metabolite of SCMC. (C) 2004 Elsevier Inc. All rights reserved.