Human phenylalanine monooxygenase and thioether metabolism

What is it about?

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. 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 L-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. Phenylalanine monooxygenase may play a wider role outside intermediary metabolism in the biotransformation of dietary-derived substituted cysteines and other exogenous thioether compounds.

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Photo by Hush Naidoo on Unsplash

Photo by Hush Naidoo on Unsplash

Why is it important?

This is the first reported paper of the role of human phenylalanine $-monooxygenase in the S-oxidation of thioether xenobiotics.