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Alkylguanidines as Inhibitors of K⁺ Transport in Isolated Barley Roots

Instituto de Biología, Universidad Nacional Autonóma de México, México, D. F., México

It has been shown that plants can accumulate K⁺ through an energy-dependent process. The effect of alkylguanidines, in particular octylguanidine on the uptake of ⁸⁶Rb⁺ by excised barley roots (Hordeum vulgare var. Apizaco LV-72), has been studied. ⁸⁶Rb⁺ was used as tracer of K⁺. The uptake of ⁸⁶Rb⁺ which is linear with time and shows saturation kinetics is inhibited by octylguanidine. Half-maximal inhibition of ⁸⁶Rb⁺ uptake is attained at 50 µM octylguanidine. Octylguanidine induces a decrease in the V_max of the process and increases the Km of the system for Rb⁺. When the effects of various alkylguanidines were studied, the following order of effectiveness was encountered; octylguanidine = hexilguanidine > butylguanidine > ethylguanidine > guanidine. This suggests that guanidines inhibit Rb⁺ uptake by interacting through its positively charged guanidinium group with a Rb⁺ carrier while the alkyl chain interacts with the hydrophobic milieu of the membrane.