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Alkylguanidine Inhibition of Ion Absorption in Oat Roots ¹

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

The effect of various alkylguanidines on ion absorption and energy metabolism in oat (Avena sativa cv. Goodfield) roots has been investigated. Of several alkylguanidines tested, octylguanidine was the most effective inhibitor of both K⁺ and Cl^– absorption by excised roots. At 225 µM octylguanidine, the transport of both ions was inhibited within 60 seconds and to a similar extent. Octylguanidine inhibited mitochondrial oxidative phosphorylation and mitochondrial adenosine 5'-triphosphatase (ATPase). The plasma membrane ATPase was also inhibited if the membranes were diluted and pretreated with Triton X-100.

Concentrations of octylguanidine giving half-maximal inhibition of K⁺ influx, mitochondrial ATPase, oxidative phosphorylation, and plasma membrane ATPase were 50, 50, 100, and 275 µM, respectively. With increasing chain length, alkylguanidines (225 µM) became progressively more inhibitory to K⁺ absorption and to the mitochondrial ATPase. Shorter chain guanidines slightly inhibited the plasma membrane ATPase, however, these compounds produced a slight stimulation in oxidative phosphorylation.

Conditions of Triton treatment that were important in the elimination of permeability barriers of plasma membrane vesicles to ATP, Mg²⁺, KCl, and octylguanidine were: concentration of Triton during pretreatment and in the assay media, concentrations of sucrose and plasma membrane during Triton treatment, and temperature of Triton treatment.

Inhibition by octylguanidine of K⁺ and Cl^– absorption by excised oat roots may be due to an inhibition of either the plasma membrane ATPase or mitochondrial oxidative phosphorylation. The isolated plasma membrane did not appear to be permeable to octylguanidine since the plasma membrane ATPase was inhibited only after treating the membrane with Triton. This result indicates that the primary site of action of octylguanidine in excised root is more likely to be the plasma membrane ATPase than mitochondrial oxidative phosphorylation.

¹ This research was supported by a grant from the International Development Research Centre and National Science Foundation Grant PCM-02471A01. Journal Paper No. 6808 of the Purdue University Agricultural Experiment Station.