title: Phenylalanine hydroxylase: possible involvement in the S-oxidation of S-carboxymethyl-l-cysteine.
abstract: 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 (0.1-0.5mg/ml). The calculated K(m) 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 K(m) and V(max) 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.
authors.affiliation: Pharmaceutical Sciences Research Division, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NN, UK.
authors.forename: Boontarika
authors.initials: B
authors.lastname: Boonyapiwat
authors.forename: Ben
authors.initials: B
authors.lastname: Forbes
authors.forename: Glyn B
authors.initials: GB
authors.lastname: Steventon
mesh_terms: D000818:Animals; D002233:Carbocysteine / Q000737:chemistry / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism; D003600:Cytosol / Q000201:enzymology*; D005260:Female; D008099:Liver / Q000201:enzymology*; D010084:Oxidation-Reduction; D010087:Oxides / Q000378:metabolism; D010649:Phenylalanine / Q000378:metabolism; D010651:Phenylalanine Hydroxylase / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013237:Stereoisomerism
chemical_list: D010087:Oxides; D010649:Phenylalanine; D002233:Carbocysteine; D010651:Phenylalanine Hydroxylase; D003545:Cysteine