Human phenylalanine monooxygenase and thioether metabolism.
OBJECTIVE
The substrate specificity of wild-type human phenylalanine monooxygenase (wt-hPAH) has been investigated with respect to the mucoactive drug, S-carboxymethyl-L-cysteine and its thioether metabolites. The ability of wt-hPAH to metabolise other S-substituted cysteines was also examined.


METHODS
Direct assays of PAH activity were by HPLC with fluorescence detection; indirect assays involved following disappearance of the cofactor by UV spectroscopy.


RESULTS
wt-hPAH catalysed the S-oxygenation of S-carboxymethyl-L-cysteine, its decarboxylated metabolite, S-methyl-L-cysteine, and both their corresponding N-acetylated forms. However, thiodiglycolic acid was not a substrate. The enzyme profiles for both phenylalanine and S-carboxymethyl-L-cysteine showed allosteric kinetics at low substrate concentrations, with Hill constants of 2.0 and 1.9, respectively, for the substrate-activated wt-hPAH. At higher concentrations, both compounds followed Michaelis-Menten kinetics, with non-competitive substrate inhibition profiles. The thioether compounds, S-ethyl-L-cysteine, S-propyl-L-cysteine and S-butyl-L-cysteine were all found to be substrates for phenylalanine monooxygenase.


CONCLUSIONS
Phenylalanine monooxygenase may play a wider role outside intermediary metabolism in the biotransformation of dietary-derived substituted cysteines and other exogenous thioether compounds.
Boonyapiwat
Boontarika
B
Bureau of Drug and Narcotic, Department of Medical Sciences, Ministry of Public Health, Nonthaburi, Thailand.
Panaretou
Barry
B
Forbes
Ben
B
Mitchell
Stephen C
SC
Steventon
Glyn B
GB
D002233:Carbocysteine / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003545:Cysteine / Q000031:analogs & derivatives / Q000378:metabolism; D004789:Enzyme Activation; D005453:Fluorescence; D006801:Humans; D007700:Kinetics; D008244:Lysophosphatidylcholines / Q000378:metabolism; D010651:Phenylalanine Hydroxylase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet; D013379:Substrate Specificity; D013440:Sulfides / Q000737:chemistry / Q000378:metabolism*; D015262:Xenobiotics / Q000737:chemistry / Q000378:metabolism
D008244:Lysophosphatidylcholines; D011994:Recombinant Proteins; D013440:Sulfides; D015262:Xenobiotics; D002233:Carbocysteine; C008425:S-methylcysteine; D010651:Phenylalanine Hydroxylase; D003545:Cysteine