Characterization and localization of laccase forms in stem and cell cultures of sycamore

Summary A laccase‐type polyphenoloxidase (EC 1.10.3.2.), abundantly secreted by suspension‐cultured sycamore (Acer pseudoplatanus) cells was purified to homogeneity. This laccase form is a glycoprotein (molecular weight 110000) with high mannose and complex glycans. The polypeptide moiety has a molecular weight of 66 000, indicating that the glycoprotein is 40% carbohydrate. Laccase is abundantly present in both the cell wall and the culture medium of suspension‐cultured sycamore cells, but it is not detected in the cytoplasm, indicating that this large protein is efficiently secreted by the cells. Polyclonal rabbit antiserum was raised against the deglycosylated protein and was used to probe extracts of sycamore stem tissues. A second laccase form (molecular weight 56 000), antigenically related to laccase from cell cultures, is abundant in the epidermis of sycamore stems. In addition, this 56 kDa laccase form co‐localizes with lignin precursors on tissue prints from sycamore stems. A polypeptide (molecular weight 50 000‐56 000), antigenically related to sycamore laccase, was also immunodetected in most plant organs previously described in the literature as polyphenoloxidase‐rich.