Über die Verwendung von 86Rb als Indikator für Kalium, Untersuchungen am lichtgeförderten 42K/K- und 86Rb/Rb-Influx bei Elodea densa/ On the Application of 86Rb as a Tracer for Potassium, Measurements of the Lightdependent 42K/K- and 86Rb/Rb-Influx in Elo

1. The light-dependent influx of K⊕ and of Rb⊕ into isolated leaves of Elodea densa was measured by using 42K⊕ and 86Rb⊕ as tracers. 2. If 86Rb was used as a tracer for K⊕ and then the K⊕ influx was determined from the specific activity σ86Rb/K an influx vK(86Rb) was obtained which was lower than the K(42K) influx (fig. 1). The ratio of vK(42K)/vK(86Rb) = SK,Rb increased with increasing [Rb⊕]0 and decreasing [K⊕]0 (figs. 1 and 7). The depression of vK(86Rb) as compared to vK(42K) was not a consequence of the competitive inhibition of K⊕ influx by Rb⊕ (cf. fig. 1). 3. As a working hypothesis it is proposed that 86Rb⊕ functions only as a tracer of Rb⊕ and that vK(84Rb) can be calculated from the parameters of the Rb (86Rb) influx and its competitive inhibition by K⊕. 4. By using the Michaelis - Menten - Kinetics a formula was derived for vK(86Rb), the Michaelis constant KmK(86 Rb), and for VmaxK(86 Rb). From these calculations it may be predicted a) that vK(86Rb) will show a Michaelis - Menten - Kinetic only if [Rb⊕] is kept constant at all K⊕ concentrations; b) that VmaxK(86 Rb) is independent of [Rb⊕] and not necessarily equal to VmaxK(42 K); and c) that KmK(86 Rb) is greater than KmK(42 K) and dependent on [Rb⊕]. 5. In order to test the calculations the mutual competitive inhibition of K⊕ - and Rb⊕ -influx (figs. 2, 3) was measured. From the Michaelis - Menten - Parameters of the Rb (86Rb) influx and the inhibition constant KiK of potassium vK(86Rb) was calculated. The predictions mentioned above were verified. The calculated depression of vK(86Rb) as compared to vK(42K) corresponded satisfactorily to the experimental values (fig. 7). 6. The isotherms of K (42K) and of Rb (86Rb) influx were measured (fig. 4). In the range of low concentrations (system 1) VmaxK(42 K) greatly exceeded VmaxRb(86 Rb), the same was true of KmK(42 K) as compared to KmRb(86 Rb). Thus in system 1 the Rb⊕ influx proceeded with much greater affinity but lower capacity than the K⊕ influx. Thus, although with regard to competitive inhibition K⊕ and Rb⊕ seem to use the same transfort site the relative similarity of vK(42K) and vK(86Rb) must be termed rather incidental though predictable. 7. At higher concentrations (system 2) the influx of K⊕ and Rb⊕ was less different with respect to Km and Vmax (fig. 7), an observation which was confirmed by competitive inhibition on almost equal terms. 8. It must be stated that the value of 86Rb⊕ as a tracer instead of 42K⊕ is limited. However, the metabolic linkages of K⊕ influx can also be detected by the use of 86Rb⊕.