CALCIUM ACTIVATION OF MAIZE ROOT PHOSPHOLIPASE-D

Calcium activation of plant lipases which occurs at relatively high concentrations is poorly understood. The effects of divalent cations on the activities of the soluble and membrane-bound phospholipase D were compared. Both the soluble and membrane-bound activities were stimulated by 1 mM Ca and to a lesser extent by 1 mM Ba, but not by 1 mM Mg, Mn, Zn, or Cd. The two enzyme forms differ in the concentration dependence of Ca activation. The membrane-bound activity exhibited some activity in the absence of Ca, whereas no activity was observed with the soluble enzyme without Ca. Maximal activation was observed at micromolar concentration with the membrane-bound enzyme compared to millimolar levels for the soluble enzyme. The effects of Ca on the structure of substrate vesicles for the soluble enzyme were determined by following changes in fluorescence of incorporated probes. Calcium had little effects on membrane structure at micromolar concentration, but alter the bilayer structure at millimolar concentrations. Thus, there was a strong association between the concentration dependence of the soluble phospholipase D activity and changes in membrane structure.