Keith Bahjat writes (re propidium toxicity)- >How dangerous is a membrane impermeable dye?? Unless it can resurrect dead >epithelial cells, it should not have access to the DNA of living cells, and >thus shouldn't be much of a safety hazard. > >We always wipe everything down with ethanol, but I've yet to see data >showing membrane impermeable DNA intercalating dyes cause problems for >living cells. I think the MSDS claims are based on a general fear of >anything that has the word "DNA" in it. > >Anyone have data to contradict this?? The funny thing is that, according to data on Molecular Probes' safety sheets, the LD50 in mice for the propidium, which is supposedly membrane-impermeant, is lower than that for ethidium, which is membrane-permeant (ethidium is usually pumped out of viable cells, which is why it often seems to work in dye-exclusion viability tests). So, either propidium isn't membrane-impermeant all the time, or it may be metabolized in vivo to something which is. Note that the terms I use here with respect to the dyes are permeant and impermeant, describing things which get through and don't, as opposed to permeable and impermeable, describing membranes which do and don't let things get through. Membranes are normally impermeable to propidium, which is impermeant to intact membranes. The other ringer in this game is that there are circumstances under which cells become transiently permeable to propidium and structurally similar dyes (e.g., TO-PRO-1 and TO-PRO-3) without loss of viability. This happens during scrape loading of cells with various normally impermeant compounds, during electroporation and other treatments used in transfection, and, in at least some bacteria, after exposure to sublethal concentrations of some antibiotics (Novo et al, Antimicrobial Agents and Chemotherapy 44:827-34, 2000). The bacterial response is interesting because it may provide a back door through which compounds with minimal host toxicity may be introduced to kill otherwise antibiotic-resistant organisms. If host cell permeability is variable in the normal scheme of things, this therapeutic approach becomes harder to implement, but we're still looking into it. All that said, while I wouldn't substitute propidium for paprika, I wouldn't get paranoid about it, either. -Howard
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