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PubMed1982

Multiple binding of thallium and rubidium to potassium-activated yeast aldehyde dehydrogenase. Influences on tertiary structure, stability and catalytic activity.

Univalent cation activators of aldehyde dehydrogenase have dual effects, both interpreted as cation-induced or -stabilized conformation changes. These two processes are differentiated by the time scales of their associated changes in activity. Using Tl+ as an activator, under certain conditions, the slower change in activity saturates at a Tl+ concentration which is only 0.1 Ks for the faster change. This, together with evidence for cation-induced rather than cation-stabilized conformation changes, is used to propose separate binding sites for cations responsible for the two activation processes. Equilibrium dialysis indicates 4 binding sites per active site for Rb+ or 6 sites for Tl+. At least one of the additional sites for Tl+ is an inhibitory site which has been differentiated from the activator sites on the basis of steady-state and pre-steady-state kinetic data.

Aldehyde Dehydrogenase↗
PubMed1983

Re-assessment of ammonium-ion affinities of NADP-specific glutamate dehydrogenases. Activation of the Neurospora crassa enzyme by ammonium and rubidium ions.

The NADP-specific glutamate dehydrogenase of Neurospora crassa shows complex interactions with NH4+ ions, characterized by biphasic downwardly convex double-reciprocal plots. These kinetics are explained by the action of NH4+ both as a substrate and, acting at a separate cation-binding site, as an activator. Rb+ ions, and to a smaller extent other univalent cations, also activate by acting as analogues of NH4+. Previous failure to recognize this effect, which probably also occurs in homologous enzymes from some other species, has led to significant overestimates in published reports of the Km for NH4+ of some NADP-specific glutamate dehydrogenases.

Ammonium Chloride↗
PubMed1999

Are the states that occlude rubidium obligatory intermediates of the Na(+)/K(+)-ATPase reaction?

In the Albers-Post model, occlusion of K(+) in the E(2) conformer of the enzyme (E) is an obligatory step of Na(+)/K(+)-ATPase reaction. If this were so the ratio (Na(+)/K(+)-ATPase activity)/(concentration of occluded species) should be equal to the rate constant for deocclusion. We tested this prediction in a partially purified Na(+)/K(+)-ATPase from pig kidney by means of rapid filtration to measure the occlusion using the K(+) congener Rb(+). Assuming that always two Rb(+) are occluded per enzyme, the steady-state levels of occluded forms and the kinetics of deocclusion were adequately described by the Albers-Post model over a very wide range of [ATP] and [Rb(+)]. The same happened with the kinetics of ATP hydrolysis. However, the value of the parameters that gave best fit differed from those for occlusion in such a way that the ratio (Na(+)/K(+)-ATPase activity)/(concentration of occluded species) became much larger than the rate constant for deocclusion when [Rb(+)] <10 mM. This points to the presence of an extra ATP hydrolysis that is not Na(+)-ATPase activity and that does not involve occlusion. A possible way of explaining this is to posit that the binding of a single Rb(+) increases ATP hydrolysis without occlusion.

Adenosine Triphosphate↗
PubMed1985

Rubidium transport in X-irradiated human erythrocytes.

Outflow of 86Rb, a radioactive analogue of potassium, from human erythrocytes X-irradiated in vitro was studied with the following results. (1) The 86Rb level in the supernatants of irradiated and control cell suspensions reflected mainly 86Rb outflow and much less its active re-uptake. (2) The effect of irradiation on 86Rb outflow was more pronounced at a low temperature (4 degrees C) than at 37 degrees C; the lowest dose of X-radiation exhibiting a significant effect on 86Rb outflow at 4 degrees C was 2.5 Gy. (3) K/Rb exchange did not seem to play an appreciable role in radiation-induced 86Rb outflow. (4) Calcium and its accumulation in irradiated cells was not found to be the cause of the effect of radiation on 86Rb outflow. (5) The effect of radiation on 86Rb outflow was higher in low Na medium but it was not inhibited by bumetanide. Rb/Na counter- or co-transport do not therefore seem to be involved in radiation-induced Rb+ outflow.

Biological Transport↗
PubMedDate not supplied

Rubidium uptake is increased in shore crabs, Carcinus maenas, acclimated to dilute seawater.

In dilute seawater, Carcinus maenas hyperosmoregulates by actively absorbing Na, K, and Cl. Here we characterize K uptake using a novel technique. Rb was used as a tracer for K transport, and hemolymph Rb levels were measured using cation chromatography. Hemolymph Rb was detectable at 0.1 mmol L(-1), which enabled determination of initial rate of Rb uptake. Crabs maintained for 3 wk in dilute artificial seawater (35% ASW crabs) maintained Na and K above the level of the external media and had elevated Na-K-ATPase activity in the posterior gills. In assay conditions matched to 100% ASW, Rb uptake was the same in 35% ASW crabs (0.45+/-0.04 micromol g(-1) h(-1)) and in crabs acclimated to normal seawater (100% ASW crabs, 0.49+/-0.05 micromol g(-1) h(-1)). In assay conditions matched to 35% ASW, Rb uptake was greater in 35% ASW crabs (0.28+/-0.03 micromol g(-1) h(-1)) compared with 100% ASW crabs (0.10+/-0.04 micromol g(-1) h(-1)). Low external [Rb] or reduced salinity were found to contribute independently to the difference between 100% ASW and 35% ASW crabs. Thus, whole-body Rb uptake in crabs can be measured by cation chromatography, and Rb uptake is greater in 35% ASW crabs than in 100% ASW crabs.

Adaptation, Physiological↗
PubMed2004

Nonradioactive rubidium ion efflux assay and its applications in drug discovery and development.

The recent sequencing of the human genome has created comprehensive information of all potential drug targets. Based on current estimations for the total number of genes, around 400 poreforming ion channel genes can be expected corresponding to about 1.3% of the human genome. Since many ion channels are involved in diseases and the currently marketed drugs act only on a small fraction of these pore-forming membrane proteins, there is a big opportunity for innovative ion channel drug discovery. In fact, recent advances in the development of functional ion channel assays are currently enabling a more systematic exploitation of this important target class. In particular, fluorescence-based methods, automated electrophysiology, and ion flux assays are most important in this regard. This article will briefly describe these methods focusing on the nonradioactive Rb(+) efflux assay that I developed in the early 1990s since it has found widespread application in drug discovery and development and greatly displaced (86)Rb(+) assays for the analysis of K(+) and nonselective cation channels in the pharmaceutical industry.

Animals↗
PubMed1997

The effects of elevated glucose on Na+/K(+)-ATPase of cultured bovine retinal pigment epithelial cells measured by a new nonradioactive rubidium uptake assay.

The effects of stimulated hyperglycemia on the Na+/K(+)-ATPase activity of cultured bovine retinal pigment epithelial (RPE) cells were investigated. Total Rb+ uptake, measured by a chromatographic method, was decreased 20-30% by 55.5 mM glucose relative to 5.55 mM glucose for culture periods of 2 to 28 days. An acute hyperglycemic stress (< 1 week) had no effect on ouabain-inhibition of Rb+ uptake or ouabain binding to RPE cells (IC50 = 55 nM for both processes) and did not alter the IC50 value (near 10 nM) for binding of strophanthidin, another selective Na+/K(+)-ATPase inhibitor. A small increase in the apparent K(m) of Rb+ for Na+/K(+)-ATPase accompanied the decrease in maximal Rb+ uptake at 55.5 mM glucose. The continuous presence of AL-1576, an aldose reductase inhibitor (ARI), normalized the effect of severe hyperglycemia on Rb+ uptake in the chronic (28 days) but not the acute exposure protocols. Thus, decreased efficiency of Na+/K(+)-ATPase caused by chronic accumulation of intracellular sorbitol can account for previously reported functional and structural alterations in the RPE cell layer of diabetic rodents. The results of the present study suggest that hyperglycemia-induced loss of Na+/K(+)-ATPase function in RPE cells, which responds to aldose reductase inhibitor treatment, contributes to the pathogenesis of diabetic retinopathy.

Aldehyde Reductase↗
PubMed1976

Rubidium release: a rapid and sensitive assay for amphotericin B.

A new assay for amphotericin B has been devised with use of the release of a radioactive potassium analogue (86rubidium; 86Rb) from Candida parapsilosis. The 86Rb-release assay is rapid, highly sensitive, and reproducible. Amphotericin B levels of 0.06-2.0 mug/ml can be measured in serum, whereas as little as 0.015 mug of amphotericin B/ml can be detected in spinal fluid. The greater sensitivity of the assay in spinal fluid as compared with that in serum may be the result of binding of the amphotericin molecules by serum components, thus decreasing the amount of amphotericin free to interact with the fungal membrane.

Amphotericin B↗