Abstract
The properties of ATPase activity in a microsomal fraction from etiolated hypocotyls of Cucumis sativus L. were investigated. The ATPase required Mg 2+ , 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 .
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