Regulation of the Formation of Acid Phosphatases by Inorganic Phosphate in Aspergillus ficuum

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Abstract
Two types of extracellular acid phosphatases are synthesized by Aspergillus ficuum NRRL 3135: a nonspecific orthophosphoric monoester phosphohydrolase (EC 3.1.3.2) with an optimum p H of 2.0, and an enzyme with restricted specificity, a mesoinositol-hexaphosphate phosphohydrolase (EC 3.1.3.8; phytase) with an optimum p H of 5.5. Although the p H 5.5 enzyme is termed a phytase, both enzymes hydrolyze phytin. Synthesis of the enzymes is repressed by high orthophosphate concentrations in the fermentation medium. The highest total level for each enzyme is synthesized in low orthophosphate medium. In high orthophosphate medium, more p H 5.5 enzyme is produced than p H 2.0 enzyme. In low orthophosphate medium, more p H 5.5 enzyme is produced than p H 2.0 enzyme during the early stages of growth, but the reverse occurs after 5 days. The enzymes are differentiated by heat denaturation at acid and alkaline p H levels. They are separated into two distinct fractions on Sephadex G-100 followed by carboxymethylcellulose column chromatography. This indicates that the two enzymes are structurally different. The K m for both enzymes is 1.25 m m when calcium phytate is the substrate. Orthophosphate competitively inhibits the p H 2.0 ( K i = 1.1 × 10 −2 m ) but not the p H 5.5 phosphatase. Neither enzyme is denatured by 50% (w/v) urea or inhibited by 0.01 m tartrate. Thus, they differ from human prostatic phosphatase.