Der Einfluß von Natrium- und Kaliumionen auf die Phosphataufnahme bei Ankistrodesmus braunii

Abstract

The uptake of phosphate as influenced by sodium and potassium ions was investigated in the light and in the dark. It was found to be a function of the external phosphate concentration. At a low concentration (up to 10⁻⁵ mol/l) in the presence of Na⁺ phosphate is quickly absorbed and hence phosphate is the limiting factor for further labelling. In the presence of K⁺ phosphate uptake is constant over a long period. The enhancement of phosphate uptake by Na⁺ is also found when the external concentration of P is raised up to 10⁻⁴ mol/l. Then the gross uptake proceeds over six hours, with the greatest Na⁺-dependent increase occurring in the label of the TCA-insoluble phosphate fraction (P_u). The phosphate uptake is strongly dependent on the pH of the reaction mixture. In the presence of Na⁺ it is highest between pH 5.6 and 7. As the uptake in the presence of K⁺ parallels the dissociation curve of the dihydrogen form H₂PO ₄ ⁻ , the Na⁺-enhancement is optimal in the alkaline pH range (pH 8). On the basis of a comparison between the pH-dependence of phosphate uptake and the dependence of the uptake on the external phosphate concentration analysed by a method of enzyme kinetics, it is suggested that Ankistrodesmus metabolically transports H₂PO ₄ ⁻ but not HPO ₄ ⁼ . Moreover, it is concluded from the absence of light stimulation and the weak inhibition of the uptake by DCMU or CCCP in the presence of K⁺ that at low P-concentrations the diffusion is limiting the uptake. Only at higher concentrations is an active phosphate uptake measured. Furthermore it is concluded that the observed Na⁺-stimulation of the ³²P-labelling of the TCA-soluble and insoluble compounds inside the cell is indirect and depends only on the action of Na⁺ and K⁺ ions at the first transport site in the plasmalemma.