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In isolated rat hepatocytes, many chemicals elicit toxicity which is inhibitable by antioxidants such as butylated hydroxytoluene (BHT). Although BHT protection is evident at concentrations of less than about 50 nmol/mg protein, higher concentrations exhibit intrinsic concentration-dependent toxicity, which involves mitochondrial dysfunction. We evaluated the possibility that both chemoprotection and intrinsic toxicity could be explained by a common mechanism involving alterations in the physical properties of cellular membranes. In the red blood cell (RBC) osmotic fragility assay, BHT at less than 60 nmol/mg protein protected against osmotic fragility; however, BHT at higher concentrations enhanced osmotic fragility such that total osmolysis occurred at 135 nmol/mg. The BHT-mediated alterations in osmotic fragility correlated with changes in membrane fluidity, determined by fluorescence polarization of the hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene. Protection from osmolysis correlated with decreased fluidity, while enhanced RBC fragility correlated with increased fluidity. In rat hepatocyte suspensions, high BHT concentrations also permeabilized the plasma and mitochondrial membranes to enzyme leakage, and these effects were accompanied by enhanced membrane fluidity. Although other mechanisms may be operative, alterations in membrane fluidity appear to be, in part, responsible for the observed chemoprotective effects at low concentrations, and intrinsic toxicity at higher concentrations of BHT.

The mixture of compounds called compound 48/80 had been shown to have antimicrobial activity against a wide variety of microorganisms. In this paper it is shown that its primary site of attack appears to be on the membrane of the cell. In its presence, Tetrahymena became much more sensitive to osmotic stress, and alpha-methylglucose was rapidly released from preloaded Escherichia coli cells. The drug also had effects on cell viability, respiration, cell division, and the release of material absorbing at 260 nm. In general, its effects paralleled those of polymyxin B, although its structure is quite different except for the presence of amino groups and hydrophobic regions in both molecules. The activity of 48/80 was not due to detergent-like, surface-active properties and was antagonized by magnesium and other cations and by phosphatidylserine. Purification of the active principle might provide a relatively simple and readily modifiable probe of membrane function and possibly a new family of useful antimicrobial compounds.