Lipid composition determines the effects of arbutin on the stability of membranes

Arbutin (hydroquinone-beta-D-glucopyranoside) is an abundant solute in the leaves of many freezing- or desiccation-tolerant plants. Its physiological role in plants, however, is not known. Here we show that arbutin protects i solated spinach (Spinacia oleracea L.) thylakoid membranes from freeze-thaw damage. During freezing of liposomes, the presence of only 20 mM arbutin l ed to complete leakage of a soluble marker from egg PC (EPC) liposomes. Whe n the nonbilayer-forming chloroplast lipid monogalactosyldiacylglycerol (MG DG) was included in the membranes, this leakage was prevented. inclusion of more than 15% MGDG into the membranes led to a strong destabilization of l iposomes during freezing. Under these conditions arbutin became a cryoprote ctant, as only 5 mM arbutin reduced leakage from 75% to 20%. The nonbilayer lipid egg phosphatidylethanolamine (EPE) had an effect similar to that of MGDG, but was much less effective, even at concentrations up to 80% in EPC membranes. Arbutin-induced leakage during freezing was accompanied by massi ve bilayer fusion in EPC and EPC/EPE membranes. Twenty percent MGDG in EPC bilayers completely inhibited the fusogenic effect of arbutin. The membrane surface probes merocyanine 540 and 2-(6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl )amino)hexanoyl-1- hexadecanoyl-sn-glycero-3-phosphocholine (NBD-C-6-HPC) r evealed that arbutin reduced the ability of both probes to partition into t he membranes. Steady-state anisotropy measurements with probes that localiz e at different positions in the membranes showed that headgroup mobility wa s increased in the presence of arbutin, whereas the mobility of the fatty a cyl chains close to the glycerol backbone was reduced. This reduction, howe ver, was not seen in membranes containing 20% MGDG. The effect of arbutin o n lipid order was limited to the interfacial region of the membranes and wa s not evident in the hydrophobic core region. From these data we were able to derive a physical model of the perturbing or nonperturbing interactions of arbutin with lipid bilayers.