Polymerisation of Gliadin Mediated by Mushroom Tyrosinase

The oxidative polymerisation of tyrosinyl residues in wheat gliadin by mushroom tyrosinase was studied. After gliadin had been incubated with mushroom tyrosinase, conjugation and SS linking were observed by SDS-PAGE. The reaction mixture of gliadin treated with mushroom tyrosinase was then analysed by size-exclusion high-performance liquid chromatography (SE-HPLC). While the peak area of fraction 5 (MW 20-60 kDa) had decreased after gliadin had been incubated with mushroom tyrosinase for 3 h, the relative peak area of fraction 2 (MW 350 kDa-1,000kDa) had increased from 8.5% to 27.0% compared with the control. There was no change in the relative area of fractions 3 (MW 130-350 kDa), 4 (MW 60 kD-130 kDa) and 6 (MW<20 kDa) during incubation with mushroom tyrosinase. Protein-bound 5- S -cysteinyl-3, 4-dihydroxyphenylalanine (5- S cysteinyldopa) in the gliadin incubated with mushroom tyrosinase was observed to increase with increasing incubation time and amount of mushroom tyrosinase. More protein-bound 5- S cysteinyldopa was formed in the 70% ethanol-insoluble fraction than in the 70% ethanol-soluble fraction at 5°C. It is suggested that the formation of protein-bound 5- S -cysteinyldopa from the tyrosinyl and cysteinyl residues of wheat gliadin can lead to intra- and intermolecular bonding of protein.