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 MIC 11 to link covalently some amino acids (tyrosine, lysine, cysteine, an d oxidized and reduced glutathione) to the ferulic acid esterified in wheat arabinoxylans was investigated, using capillary viscometry, SE-H PLC, and RP-HPLC of phenolic and thiol compounds. The laccase was comp ared to the system hydrogen peroxide/horseradish peroxidase. Both oxid ative systems were able to gel arabinoxylan solutions by coupling feru loyl esters of adjacent chains into dehydrodimers. Cysteine and reduce d glutathione hindered gelation, whereas tyrosine, lysine, and oxidize d glutathione had no effect. Under the experimental conditions, in the presence of thiol compounds, a time delay proportional to the thiol c oncentration was observed. During this period, no esterified ferulic a cid was consumed. Cysteine was not directly oxidized either by free fe rulic acid or by laccase. When free ferulic acid, cysteine, and laccas e reacted together, cysteine was readily oxidized into cystine. Simila r results were obtained with reduced glutathione. Thus, ferulic acid o xidized by laccase into semi-quinone was regenerated by an oxidation-r eduction reaction involving thiols. No direct coupling of thiol to sem i-quinone by an addition reaction could be demonstrated.