Effect of lysine, tyrosine, cysteine, and glutathione on the oxidative cross-linking of feruloylated arabinoxylans by a fungal laccase

The potential of a laccase from the fungus Pycnoporus cinnabarinus MIC11 to link covalently some amino acids (tyrosine, lysine, cysteine, and oxidized and reduced glutathione) to the ferulic acid esterified in wheat arabinoxylans was investigated, using capillary viscometry, SE-HPLC, and RP-HPLC of phenolic and thiol compounds. The laccase was compared to the system hydrogen peroxide/horseradish peroxidase. Both oxidative systems were able to gel arabinoxylan solutions by coupling feruloyl esters of adjacent chains into dehydrodimers. Cysteine and reduced glutathione hindered gelation, whereas tyrosine, lysine, and oxidized glutathione had no effect. Under the experimental conditions, in the presence of thiol compounds, a time delay proportional to the thiol concentration was observed. During this period, no esterified ferulic acid was consumed. Cysteine was not directly oxidized either by free ferulic acid or by laccase. When free ferulic acid, cysteine, and laccase reacted together, cysteine was readily oxidized into cystine. Similar results were obtained with reduced glutathione. Thus, ferulic acid oxidized by laccase into semi-quinone was regenerated by an oxidation-reduction reaction involving thiols. No direct coupling of thiol to semi-quinone by an addition reaction could be demonstrated.