Interactions of erythrosin B (U.S. F, D & C Red 3) with rat cortical membranes

Fluorescein analogues inhibit SecA ATPase: the first sub-micromolar inhibitor of bacterial protein translocation

SecA is a central component of the general secretion system that is essential for bacterial growth and thus an ideal target for antimicrobial agents. A series of fluorescein analogues were first screened against the ATPase activity using the truncated unregulated SecA catalytic domain. Rose bengal (RB) and erythrosin B (EB) were found to be potent inhibitors SecA with IC(50) values of 0.5 μM and 2 μM, respectively. RB and EB inhibit the catalytic SecA ATPase more effectively than the F(1) F(0) -proton ATPase. We used three assays to test the effect of these compounds on full-length SecA ATPase: in solution (intrinsic ATPase), in membrane preparation, and translocation ATPase. RB and EB show the following trend in terms of IC(50) values: translocation ATPase<membrane ATPase<intrinsic ATPase. Very importantly, the potency of these fluorescein analogues in inhibiting the truncated SecA ATPase correlates with their ability to inhibit the biologically relevant protein translocation activity of SecA. The in vitro translocation of proOmpA precursors into membrane vesicles is strongly inhibited by RB with IC(50) values of approximately 0.25 μM, making RB the most potent inhibitor of SecA ATPase and SecA-dependent protein translocation reported thus far. The ability of these compounds to inhibit SecA also directly translates into antibacterial effects. Our findings show the value of fluorescein analogues as probes for mechanistic studies of SecA functions and for the potential development of new antimicrobial agents with SecA as the target.

Brain lesions induced by specific and non-specific inhibitors of sodium-potassium ATPase

The cytotoxicity in the brain of potent selective and non-selective inhibitors of Na+/K+ ATPase has been assessed. Following injection of cardiac glycosides into the dorsal hippocampus of rats, the extent of neuronal loss roughly paralleled their potencies as inhibitors of the enzyme. Dihydroouabain was less potent than ouabain as a cytotoxin by an order of magnitude, similar to their relative affinities for Na+/K+ ATPase. The non-specific inhibitors, melittin, erythrosin B and zinc (chloride) were less neurocytotoxic than the selective inhibitors having equivalent potencies. The toxicity of a low dose of ouabain appeared to be selective for neuronal perikarya as staining for acetylcholinesterase (present on the nerve terminals of the afferent cholinergic innervation) was unaffected. A higher dose of ouabain caused a non-specific necrosis including damage to the neuropil and a loss of cholinesterase staining. Concurrently, there was an invasion of the tissue by cells resembling foaming macrophages. Other inhibitors of the enzyme caused a mixed pathology with both types of responses evident. It is suggested that the pathological response may depend on the relative degrees to which the glial and neuronal activities of the enzyme are affected.

Na,K-ATPase is decreased in hippocampus of kainate-lesioned rats

The effects of intraventricular injection of kainic acid on the Na,K-ATPase (Na,K pump) were examined in discrete pyramidal cell regions of rat hippocampus. [3H]Ouabain binding was used to quantitate Na,K-ATPase catalytic subunits and in situ hybridization was used to determine Na,K-ATPase mRNA levels. Large decreases were found in both [3H]ouabain binding and alpha 3 isoform mRNA in hippocampus areas, especially in the CA3 pyramidal cell layer, which sustains heavy cell losses as a result of bilateral, intraventricular injection of kainic acid. Substantial decreases in the high affinity component of ouabain binding and in the alpha 3 isoform mRNA (but not isoforms for other Na,K-ATPase subunits) were also observed in the CA1 region of hippocampus, an area preserved in this model. High affinity [3H]ouabain binding was decreased 25-33% in the stratum pyramidale and stratum radiatum after treatment with kainic acid, and alpha 3 mRNA was decreased by 26-50%. To further characterize the decrease in alpha 3 mRNA, animals were killed at 1, 2, and 3 weeks after injection of kainate and results show a large decrease in alpha 3 mRNA only at 2 weeks recovery time. While the pathology underlying temporal lobe epilepsy is unclear, changes in the Na,K-ATPase may be involved in abnormal firing characteristics of cells in epileptic tissue.

Localization and characterization of binding sites with high affinity for [3H]ouabain in cerebral cortex of rabbit brain using quantitative autoradiography

[3H]Ouabain binding was studied in sections of rabbit somatosensory cortex by quantitative autoradiography and in rabbit brain microsomal membranes using a conventional filtration assay. KD values of 8-12 nM for specific high-affinity binding of [3H]ouabain were found by both methods. High-affinity binding was not uniformly distributed in somatosensory cortex and was localized predominantly to laminae 1, 3, and 4. [3H]Ouabain binding in tissue sections was stimulated by the ligands Mg2+/Pi or Mg2+/ATP/Na+ and was inhibited by K+ (IC50 = 0.7-0.9 mM), N-ethylmaleimide, 5,5'-dithiobis(2-nitrobenzoic acid), and erythrosin B. We conclude that [3H]ouabain is reversibly and specifically bound with high affinity in rabbit brain tissue sections under conditions that favor phosphorylation of Na+,K+-ATPase. Quantitative autoradiography is a powerful tool for assessing the affinity and number of specific ouabain binding sites in brain tissue.

Characterization of transmitter release as a response of vertebrate neural tissue to erythrosin B

A rat cerebral cortical slice preparation was used to study the response of transmitter release to the application of the food dye, Erythrosin B, a tetraiodinated derivative of fluorescein. Erythrosin B (100 microM) stimulated net release of previously taken up [3H]norepinephrine and [3H]gamma-aminobutyric acid (GABA). The Erythrosin-induced release of GABA (the only transmitter studied) occurred in the absence of added Ca2+, and in the presence of tetrodotoxin (TTX). Ultrastructural analysis of the vesicle content of frog neuromuscular junctions treated with Erythrosin B revealed a diminution in the number of synaptic vesicles present in the nerve terminal. By using fluorescein and some halogen-substituted derivatives including Erythrosin B, it was found that incubation with the unhalogenated compound caused no net release, whereas incubation with the iodine-, chlorine- or bromine-substituted compound did cause release. It was also found that somewhat greater release induced by Erythrosin B (at 100 microM) occurred in the light than in the dark. That Erythrosin B inhibits the Na+,K+,Mg2+-ATPase was confirmed in this preparation; it did so in both light and dark. The discrepancy between release and Na+,K+,Mg2+-ATPase blockade in the dark suggests that release either occurs by some other mechanism than by Na+,K+,Mg2+-ATPase blockade, or that an additional light-dependent process contributes to the release. We conclude that Erythrosin B can presumably induce net release of transmitters generally, that release does not occur via the TTX-sensitive Na+ channel, that release via vesicles does occur, and that light somewhat enhances the release.