Effects of metal elements on acid phosphatase activity in cucumber (Cucumis sativus L.) seedlings

Acid phosphatases (E.C.3.1.3.2) are a group of enzymes widely distributed in nature, which nonspecifically catalyze the hydrolysis of a variety of phosphate esters in pH ranges from 4 to 6 and play a major role in the supply and metabolism of phosphate in plants. The objective of the present study was to investigate the in vitro effects of some metals on the activity of acid phosphatase in cucumber seedlings (Cucumis sativus L.) and to determine their kinetic parameters. The enzyme was assayed with Hg, Cd, Mn, Pb, Zn, K and Na at the 0.001–1 mM range using ATP, PPi and β-glycerol phosphate as substrates. Mn, Na and Cd did not significantly alter the enzyme activity. K caused a broad activation at low concentrations and an inhibition at high concentrations (10 mM) and lead caused no inhibition. Acid phosphatase was inhibited by Hg and Zn and the inhibition type and IC₅₀ values were determined for these metals. Hg presented a mixed inhibition type with PPi and ATP as substrates and uncompetitive inhibition with β-glycerol phosphate as substrate. Zn presented competitive inhibition for ATP as substrate, and a mixed inhibition type with PPi and β-glycerol phosphate as substrate. IC₅₀ values were 0.02, 0.3 and 0.15 mM for Hg, and 0.056, 0.035 and 0.24 mM for Zn with ATP, PPi and β-glycerol phosphate as substrates, respectively. Analysis of these results indicates that Zn is a more potent inhibitor of acid phosphatase from cucumbers than Hg.