Regulatory role of polyamine in the acid phosphatase from potato tubers

Effects of polyamine and metal ions on the new type of acid phosphatase purified from potato (Solanum tuberosum L. Irish Cobbler) tubers were analyzed. The enzyme belongs to nonspecific acid phosphatase family (EC 3.1.3.2), which hydrolyzes various phosphorylated substrates. The enzyme hydrolyzed inorganic pyrophosphate as a preferred substrate, and exhibited the hyperbolic kinetics with respect to the substrate, inorganic pyrophosphate in the absence of metal cations. Polyamine activated the enzyme effectively by lowering the Km value without appreciable changes in the maximal velocity. The most effective polyamines as activators were spermine and spermidine. Mg2+ ion increased the Km value without affecting the maximal velocity of the enzyme, but Ca2+ ion decreased both the Km and Vmax values. Increasing concentrations of spermine also decreased the Km value irrespective of Mg2+ ion included, but gave a constant Km and Vmax values in the absence and presence of Ca2+ ion. Action of spermine and metal ions can be explained by the complex formation with the substrate pyrophosphate. The acid phosphatase from potato can utilize the pyrophosphate–spermine or pyrophosphate–Ca2+ complex as the preferred substrates. However, the enzyme can use the pyrophosphate–Mg complex with a weak affinity for the active site. Polyamine activates acid phosphatase in the absence and presence of metal cations, and activation by polyamine of the enzyme may contribute to the stimulation of starch biosynthesis and the control of glycolysis/gluconeogenesis by regulating PPi levels in growing potato tubers.