Decker H, Ryan M, Jaenicke E, Terwilliger N
Institute for Molecular Biophysics, University of Mainz, D55128 Mainz, Germany.
J Biol Chem. 2001 May 25;276(21):17796-9. doi: 10.1074/jbc.M010436200. Epub 2001 Feb 15.
Phenoloxidase, widely distributed among animals, plants, and fungi, is involved in many biologically essential functions including sclerotization and host defense. In chelicerates, the oxygen carrier hemocyanin seems to function as the phenoloxidase. Here, we show that hemocyanins from two ancient chelicerates, the horseshoe crab Limulus polyphemus and the tarantula Eurypelma californicum, exhibit O-diphenoloxidase activity induced by submicellar concentrations of SDS, a reagent frequently used to identify phenoloxidase activity. The enzymatic activity seems to be restricted to only a few of the heterogeneous subunits. These active subunit types share similar topological positions in the quaternary structures as linkers of the two tightly connected 2 x 6-mers. Because no other phenoloxidase activity was found in the hemolymph of these animals, their hemocyanins may act as a phenoloxidase and thus be involved in the primary immune response and sclerotization of the cuticle. In contrast, hemolymph of a more recent arthropod, the crab Cancer magister, contains both hemocyanin with weak phenoloxidase activity and another hemolymph protein with relatively strong phenoloxidase activity. The chelicerate hemocyanin subunits showing phenoloxidase activity may have evolved into a separate phenoloxidase in crustaceans.
酚氧化酶广泛分布于动物、植物和真菌中，参与许多生物学上的重要功能，包括硬化作用和宿主防御。在螯肢动物中，氧载体血蓝蛋白似乎起着酚氧化酶的作用。在此，我们表明，来自两种古老螯肢动物——鲎（美洲鲎）和狼蛛（加州红玫瑰蜘蛛）的血蓝蛋白，在亚胶束浓度的十二烷基硫酸钠（一种常用于鉴定酚氧化酶活性的试剂）诱导下，表现出邻二酚氧化酶活性。这种酶活性似乎仅局限于少数几种异源亚基。这些活性亚基类型在四级结构中具有相似的拓扑位置，作为两个紧密相连的2×6聚体的连接物。由于在这些动物的血淋巴中未发现其他酚氧化酶活性，它们的血蓝蛋白可能充当酚氧化酶，从而参与初级免疫反应和表皮的硬化作用。相比之下，一种较新的节肢动物——太平洋岩蟹的血淋巴中，既含有酚氧化酶活性较弱的血蓝蛋白，又含有另一种酚氧化酶活性相对较强的血淋巴蛋白。表现出酚氧化酶活性的螯肢动物血蓝蛋白亚基，可能在甲壳类动物中进化成了一种独立的酚氧化酶。
