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Inhibition of Adenosine Triphosphatase Activity of the Plasma Membrane Fraction of Oat Roots by Diethylstilbestrol ¹

^a Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907

Diethylstibestrol (DES) inhibited noncompetitively the ATPase in the plasma membrane fraction from Avena sativa L. cv. Goodfield roots when assayed in the presence of MgSO₄ or MgSO₄ plus KCl. In the presence of MgSO₄, 7.1x10^–5 molar DES inhibited the enzyme 50%; whereas in the presence of MgSO₄ and KCl, 1.3x10^–4 molar DES was required for the same inhibition. Dixon plots indicated that in the presence of MgSO₄, one molecule of DES bound to one molecule of ATPase; however, in the presence of MgSO₄ and KCl, two or more molecules bound to one ATPase molecule. These results suggested that KCl causes a conformational change in the enzyme which exposes additional binding sites for DES, but that these sites are not as inhibitory as the first binding site.

In addition to KCl, other factors also affected the DES inhibition of the ATPase. Plasma membrane vesicles warmed to 38 C were inhibited more than vesicles kept on ice prior to assay. DES inhibited the Triton X-100-treated ATPase less than the ATPase which was not detergent-treated. Finally, studies with DES analogs showed that the hydroxyl groups of DES were essential for inhibition and that steric configurations of the molecule were important.

DES inhibition of the ATPase suggests that DES inhibits K⁺ absorption in oat roots by inhibiting the ATPase. Inhibition of K⁺ absorption was greater than inhibition of the ATPase, and thus DES may also inhibit other aspects of metabolism that are involved with ion absorption.

¹ This research was supported by National Science Foundation Grant 31052X. Journal Paper No. 7152 of the Purdue University Agriculture Experiment Station.