Stretch sensitivity of transmembrane mobility of hydrogen peroxide through voids in the bilayer. Role of cardiolipin.

Availability of voids for diffusion of quinone in the membrane was shown to be the rate-limiting step in electron transport in mitochondria and chloroplasts (Mathai, J. C., Sauna, Z. E., John, O., and Sitaramam, V (1993) J. Biol. Chem. 268, 15442-15454). The primary role of voids in these diffusion-controlled reactions required a more rigorous documentation of the role of diffusion in membranes by independent measurements. The transbilayer diffusion of hydrogen peroxide as monitored by occluded catalase activity was developed as a kinetically valid probe to specifically address this question. This in turn led to unique results on the mechanistic basis of stretch (= hypo-osmotic) activation of hydrogen peroxide permeation via such voids. The rate of peroxide permeation is shown to be markedly stretch sensitive in some cells/organelles (e.g. peroxisomes) and insensitive in others (e.g. erythrocytes); this was equally true of liposomes prepared from lipids extracted from the corresponding cells/organelles. The molecular basis of stretch sensitivity was uncovered using specific binary mixtures of lipids: while pure phosphatidyl choline liposomes were stretch insensitive, these became sensitive when doped only with specific lipids, viz. cardiolipin and cerebrosides. Cholesterol abolished this stretch sensitivity in ternary mixtures. Induction of stretch sensitivity by cardiolipin was marked by lowering of activation energy for peroxide diffusion, a negative temperature coefficient for glucose permeation while further addition of cholesterol reversed these phenomena. The steady state fluorescence polarization studies revealed intimate correlations between anisotropy, hydrogen peroxide diffusion, and stretch sensitivity consistent with presence of voids in these binary mixtures.