An investigation of hydroethidine as a fluorescent vital stain for prokaryotes

Selective fluorescent imaging of superoxide in vivo using ethidium-based probes

The putative oxidation of hydroethidine (HE) has become a widely used fluorescent assay for the detection of superoxide in cultured cells. By covalently joining HE to a hexyl triphenylphosphonium cation (Mito-HE), the HE moiety can be targeted to mitochondria. However, the specificity of HE and Mito-HE for superoxide in vivo is limited by autooxidation as well as by nonsuperoxide-dependent cellular processes that can oxidize HE probes to ethidium (Etd). Recently, superoxide was shown to react with HE to generate 2-hydroxyethidium [Zhao, H., Kalivendi, S., Zhang, H., Joseph, J., Nithipatikom, K., Vasquez-Vivar, J. & Kalyanaraman, B. (2003) Free Radic. Biol. Med. 34, 1359-1368]. However, 2-hydroxyethidium is difficult to distinguish from Etd by conventional fluorescence techniques exciting at 510 nm. While investigating the oxidation of Mito-HE by superoxide, we found that the superoxide product of both HE and Mito-HE could be selectively excited at 396 nm with minimal interference from other nonspecific oxidation products. The oxidation of Mito-HE monitored at 396 nm by antimycin-stimulated mitochondria was 30% slower than at 510 nm, indicating that superoxide production may be overestimated at 510 nm by even a traditional superoxide-stimulating mitochondrial inhibitor. The rate-limiting step for oxidation by superoxide was 4x10(6) M-1.s-1, which is proposed to involve the formation of a radical from Mito-HE. The rapid reaction with a second superoxide anion through radical-radical coupling may explain how Mito-HE and HE can compete for superoxide in vivo with intracellular superoxide dismutases. Monitoring oxidation at both 396 and 510 nm of excitation wavelengths can facilitate the more selective detection of superoxide in vivo.

Reactive oxygen species play no role in the candidacidal activity of the salivary antimicrobial peptide histatin 5

The mechanism of action of antimicrobial peptides is still a matter of debate. The formation of ROS (reactive oxygen species) has been suggested to be the crucial step in the fungicidal mechanism of a number of antimicrobial peptides, including histatin 5 and lactoferrin-derived peptides. In the present study we have investigated the effects of histatin 5 and of a more amphipathic synthetic derivative, dhvar4, on the generation of ROS in the yeast Candida albicans, using dihydroethidium as an indicator for ROS. With both peptides, a substantial enhancement of fluorescence was observed. However, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), a cell-permeant ROS scavenger, did not have an inhibitory effect on killing or on the enhancement of fluorescence. Furthermore, antimycin and azide, which have been reported to induce ROS in vitro, were not able to enhance the dihydroethidium fluorescence, while chlorhexidine, a non-specific antiseptic agent, enhanced dihydroethidium fluorescence to the same extent as did the peptides. Fluorescence microscopy showed the fluorescence enhancement to be a consequence of the release of unbound preformed ethidium from the mitochondrial matrix within the cell. It is concluded that ROS do not play a role in the histatin 5-mediated killing of C. albicans.