Phospholipid spin probes measure the effects of ethanol on the molecular order of liver microsomes

Ethanol, in vitro, is known to perturb the molecular order of the phospholipids in biological membranes, while chronic ethanol exposure, in vivo, leads to resistance to disordering. Such changes have usually been measured by electron spin resonance, utilizing fatty acid spin probes. The use of such probes is controversial, since their orientation in the membrane may not accurately represent that of individual phospholipids. We, therefore, compared ethanol-induced structural perturbations in the membranes of rat hepatic microsomes measured with the spin probe 12-doxylstearic acid (SA 12) with those assayed with various phospholipid spin probes. With SA 12, the addition of increasing amounts of ethanol (50-250 mM) in vitro caused a progressive decrease in the membrane molecular order, as measured by electron spin resonance (ESR). By contrast, microsomes obtained from rats chronically fed ethanol were resistant to the disordering effect of ethanol. Microsomes labeled with the phospholipid spin probes 1-palmitoyl-2-(12-doxylstearoyl)phosphatidylcholine, -phosphatidylethanolamine, or -phosphatidic acid also exhibited increased disordering with the addition of increasing amounts of ethanol. However, the effect noted with phospholipid spin probes was less than that observed with the fatty acid probe. Microsomes obtained from the livers of chronically intoxicated animals labeled with the phospholipid probes were also resistant to the disordering effects of ethanol in vitro. These results suggest that fatty acid spin probes are qualitatively valid for measuring membrane perturbations in biological membranes, ethanol affects all microsomal phospholipids, regardless of chemical dissimilarities (e.g., head-group structure), in a qualitatively similar fashion, and the fluidization of fatty acyl chains in microsomal membranes is comparable in different membrane phospholipids.