Non-Enzymatic Hydroxylation of Aromatic Compounds

Phenylalanine 4-Monooxygenase and the S-Oxidation of S-Carboxymethyl-L-cysteine

The identity of the enzyme responsible for the S-oxidation of the mucolytic S-substituted L-cysteine drug, S-carboxymethyl-L-cysteine (SCMC), has been actively investigated for the last 10 years. A genetic polymorphism exists in the oxidation of the thioether moiety that has been identified as a disease susceptibility factor in a number of degenerative diseases. This polymorphism has also been implicated in the wide variation in clinical response to SCMC therapy in man. To date little is known about the molecular enzymology of this reaction but a previous investigation revealed that rat activated phenylalanine 4-monooxygenase (PAH) could S-oxidise both Met- and S-methyl-L-cysteine (SMC) to their S-oxide metabolites. We have investigated the hypothesis that SCMC was also a substrate for activated PAH in the cytosolic faction of the Wistar rat. 1. Substrate and inhibitor investigation revealed that SCMC was a substrate for activated PAH activity in vitro. 2. The large aromatic amino acid hydroxylase monoclonal antibody and the Fe3+ chelator, deferoxamine, completely inhibited both Phe and SCMC oxidation to their respective metabolites. 3. Analysis of the Dixon plots revealed that both Phe and SCMC competitively inhibited each other's oxidation. 4. Correlation studies showed that the rate of production of Tyr was positively correlated to the production of both SCMC and SMC S-oxides in 20 female Wistar rat hepatic cytosolic fractions. These results strongly support the hypothesis that PAH is the enzyme responsible for SCMC S-oxidation in the rat.

THE HYDROXYLATION OF SKATOLE

The mixture of products obtained when skatole is oxidized with the ferrous sulphate – ascorbic acid – EDTA – air system has been examined by means of paper and thin layer chromatographic techniques. 3-Methyloxindole, o-aminoacetophenone, N-formyl-o-aminoacetophenone, and the four isomeric hydroxyskatoles have been definitely identified. A number of, as yet, unidentified compounds, which react positively with Ehrlich's reagent, are also present in the reaction mixture. The mechanism of the hydroxylation reaction and its possible physiological significance are discussed.

THE HYDROXYLATION OF SKATOLE

The mixture of products obtained when skatole is oxidized with the ferrous sulphate – ascorbic acid – EDTA – air system has been examined by means of paper and thin layer chromatographic techniques. 3-Methyloxindole, o-aminoacetophenone, N-formyl-o-aminoacetophenone, and the four isomeric hydroxyskatoles have been definitely identified. A number of, as yet, unidentified compounds, which react positively with Ehrlich's reagent, are also present in the reaction mixture. The mechanism of the hydroxylation reaction and its possible physiological significance are discussed.