Characterization of Ca²⁺ Transport in Purified Endoplasmic Reticulum Membrane Vesicles from Lepidium sativum L. Roots

The characteristics of Ca(2+) transport into endoplasmic reticulum vesicles isolated from roots of Lepidium sativum L. cv Krause have been investigated. The concentration of free Ca(2+) and ATP needed for half-maximal activity were 2.5 and 73 micromolar, respectively, and the enzyme obeyed Michaelis-Menten-like kinetics. The pH maximum occurred at 7.5 and the activity was greatly reduced at either pH 7.0 or 8.0.The Ca(2+)-dependent modulation protein, calmodulin, was tested for its effect on Ca(2+) transport into endoplasmic reticulum vesicles. Although the phenothiazine inhibitors chlorpromazine, fluphenazine, and trifluoperazine all inhibited Ca(2+) transport activity with a half-maximal effect at approximately 35 micromolar, authentic bovine brain calmodulin did not alter the activity at concentrations of 0.5 to 8 micrograms per milliliter. Calmodulin also showed no influence on the time-dependent accumulation of Ca(2+) into vesicles. The membranes did not contain endogenously bound calmodulin since washing with (ethylenebis[oxyethylenenitrile])tetraacetic acid or fluphenazine, treatments which disrupt calmodulin binding, did not alter Ca(2+) transport activity. The inhibition of Ca(2+) transport by phenothiazine drugs was likely related to their nonspecific interaction with the membrane. Thus, there was no indication that calmodulin regulated Ca(2+) uptake into root endoplasmic reticulum.