Characterization of a Light-modulated ATPase Activity in a Microsomal Fraction from Cucumber Hypocotyls

The properties of ATPase activity in a microsomal fraction from etiolated hypocotyls of Cucumis sativus L. were investigated. The ATPase required Mg²⁺, was stimulated by K⁺ and was partially inhibited by vanadate. The effect of K⁺ was unspecific and suggested that, at the tested concentration (50 mM), K⁺ stimulated ATPase activity by facilitating H⁺ extrusion by the ATPase and in turn its further activity. This assumption was supported by the effect of the K⁺ and pH gradients on K⁺ stimulation. The stimulation by K⁺ was increased by red light treatments, while red irradiation in vivo followed by far red irradiation in vitro was less stimulatory. This implies that phytochrome modulates the K⁺ stimulation. Ionophores such as gramicidin and nigericin also stimulated the ATPase activity in the presence of K⁺. This effect was prominent only after far red irradiation in vitro.