Specific inhibitor and substrate specificity of alkaline phosphatase expressed in the symbiotic phase of the arbuscular mycorrhizal fungus, Glomus etunicatum

Abstract

Specific inhibitor and substrate specificity of alkaline phosphatase in the arbuscule of Glomus etunicatum were investigated, and the possible role of this enzyme in the symbiosis was discussed. Mycorrhizal roots of marigold (Tagetes patula) were digested by cellulase and pectinase to separate the intraradical hyphae from the root tissue, and phosphatase activity was stained at pH 8.5 and 5.0. The activity of alkaline phosphatase (pH 8.5) in arbuscules was inhibited in the presence of beryllium, whereas that of acid phosphatase (pH 5.0) was less sensitive to beryllium. Specificity and effectiveness of beryllium on the alkaline phosphatase was further confirmed using fractionated (soluble and insoluble) enzyme prepared from the separated hyphae. The soluble and insoluble alkaline phosphatases hydrolyzed phosphomonoester compounds (glucose-6-phosphate, β-glycerophosphate, trehalose-6-phosphate and glucose 1-phosphate) but not pyrophosphate compounds (ATP and polyphosphate) which were hydrolyzed by acid phosphatase efficiently. The insoluble alkaline phosphatase showed high specific activity (on a protein basis) and high sensitivity to beryllium. Kinetic analysis of the insoluble alkaline phosphatase suggested the involvement of this enzyme in the sugar metabolism of the fungus due to lower Km values for sugar phosphate such as glucose-6-phosphate and treha-lose-6-phosphate.