Characterization and role of tyrosinases in the lichen Lobaria pulmonaria (L.) Hoffm.

Tyrosinases are a widespread family of multicopper oxidase enzymes. In our earlier work, we identified the presence of tyrosinases in lichenized Ascomycetes based on their substratum specificity, sensitivity to inhibitors and molecular mass. Here, we present a more detailed characterization of a tyrosinase from the lichen Lobaria pulmonaria. We also compare tyrosinase activity with the activities of laccases and peroxidases, the other redox enzymes present in this species. The importance of tyrosinases in lichen biology was studied by testing their role in melanin synthesis. Laboratory experiments clearly showed that tyrosinases from L. pulmonaria resemble those from other lichens and in free-living fungi. While the tyrosinases can metabolize the melanin precursor L-DOPA, L-DOPA can also be metabolized by peroxidases and laccases. A field experiment showed that exposing shade-adapted L. pulmonaria to normal solar radiation induces L-DOPA melanin synthesis. Synthesis occurred when lichens were exposed to either direct sunlight, or placed under a wavelength-neutral filter that slightly reduced overall light. In lichens receiving unfiltered sunlight, melanin synthesis was accompanied by increased laccase activity; by contrast, no changes in enzyme activity occurred in lichens placed under the wavelength-neutral filter. Melanization was reduced by placing lichens under filters that removed UV-B, and prevented by filters that removed both UV-A and UV-B. Removing UV-B had no effect on enzyme activity, whereas removing both UV-A and UV-B increased tyrosinase activity. Results from this study indicate that under some conditions laccases may be involved in melanin synthesis, but they provide no evidence for a role for tyrosinases in melanization. Although high tyrosinase activities are widespread in lichens, many questions on the role of this enzyme in lichen biology remain to be answered.