Effect of ionic concentration on the higher-order structure of prophenol oxidase in Drosophila melanogaster

Phenol oxidase in Drosophila melanogaster occurs as precursors designated prophenol oxidases A1 and A3. Crossing experiments between isozyme variants proved that prophenol oxidase in this species is a homodimer. Prophenol oxidases were partially purified using ammonium sulfate fractionation, phenyl Sepharose, and DEAE-cellulose column chromatography. The preparations were mixed, then dialyzed against buffer containing varying salt concentrations. The resulting prophenol oxidase was analyzed by gel electrophoresis. At 20 mM KCl or NaCl, two bands of phenol oxidase were observed, corresponding to the parental ones as monomer, whereas at 200 mM KCl or NaCl, three bands appeared in the gel, one being a dimer. The monomer-dimer reversibility of the Drosophila prophenol oxidase depends on the salt concentrations. The phenol oxidase activity remained unaffected within the KCl concentrations tested. Considering the ionic concentration of Drosophila hemolymph, these results indicate that prophenol oxidase exists as a dimer in vivo, and the higher-order structure of prophenol oxidase can be altered reversibly by ionic concentrations in vitro.