Co-occurrence of the multicopper oxidases tyrosinase and laccase in lichens in sub-order peltigerineae.

BACKGROUND AND AIMS: Following previous findings of high extracellular redox activity in lichens and the presence of laccases in lichen cell walls, the work presented here additionally demonstrates the presence of tyrosinases. Tests were made for the presence of tyrosinases in 40 species of lichens, and from selected species their cellular location and molecular weights were determined. The effects of stress and inhibitors on enzyme activity were also studied.
METHODS: Tyrosinase and laccase activities were assayed spectrophotometrically using a variety of substrates. The molecular mass of the enzymes was estimated using polyacrylamide gel electrophoresis. KEY
RESULTS: Extracellular tyrosinase and laccase activity was measured in 40 species of lichens from different taxonomic groupings and contrasting habitats. Out of 20 species tested from the sub-order Peltigerineae, all displayed significant tyrosinase and laccase activity, while activity was low or absent in other species tested. Representatives from both groups of lichens displayed low peroxidase activities. Identification of the enzymes as tyrosinases was confirmed by the ability of lichen thalli or leachates derived by shaking lichens in distilled water to metabolize substrates such as L-dihydroxyphenylalanine (DOPA), tyrosine and epinephrine readily in the absence of hydrogen peroxide, the sensitivity of the enzymes to the inhibitors cyanide, azide and hexylresorcinol, activation by SDS and having typical tyrosinase molecular masses of approx. 60 kDa. Comparing different species within the Peltigerineae showed that the activities of tyrosinases and laccase were correlated to each other. Desiccation and wounding stimulated laccase activity, while only wounding stimulated tyrosinase activity.
CONCLUSIONS: Cell walls of lichens in sub-order Peltigerineae have much higher activities and a greater diversity of cell wall redox enzymes compared with other lichens. Possible roles of tyrosinases include melanization, removal of toxic phenols or quinones, and production of herbivore deterrents.