Membrane structure and the tenuously maintained resistance to staining with Nε-dansyl-l-lysine shown by many cells

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Abstract
The ability to resist staining by Nε-dansyl-l-lysine is tenuously maintained in the majority of live nucleated cells taken from tissues concerned with immune function. Resistance is lost under a variety of nonphysiological conditions known to, or likely to, cause protein denaturation or aggregation. In contrast to that of dansyl-γ-aminobutyrate, the fluorescence intensity of Nε-dansyl-l-lysine is only weakly enhanced by native proteins. This is further reduced on denaturation or aggregation of the proteins. It is unlikely, therefore, that cellular uptake of, and staining by, Nε-dansyl-l-lysine is a direct consequence of membrane protein denaturation/aggregation but may result from a decrease in protein-phospholipid interactions leading to formation of phospholipid domains. Previous work has indicated that such features are stained by Nε-dansyl-l-lysine (Humphries, G.M.K., Lovejoy, J.P., 1983,Biophys. J. 42:307–310; Humphries, G.M.K., Lovejoy, J.R., 1983,Biochem. Biophys. Res. Commun. 111:768–774). Although it appears likely that passage through a dansyl-lysine-staining state is a common, if not universal, prelude to cell death (as monitored by uptake of trypan blue), not all cells that lose resistance to dansyl-lysine staining are moribund. Resistance to staining is also lost by macrophages on binding to solid substrates and multivalent ligands. The possible physiological significance of this is discussed.