Simulation of dehydration injury to membranes from soybean axes by free radicals

Smooth microsomal membranes were isolated from axes of soybean (Glycine max L. Merr.) seeds at the dehydration-tolerant (6 hours of imbibition) and dehydration-susceptible (36 hours of imbibition) stages of development and were exposed to free radicals in vitro using xanthine-xanthine oxidase as a free radical source. Wide angle x-ray diffraction studies indicated that the lipid phase transition temperature of the microsomal membranes from the dehydration-tolerant axes increased from 7 to 14 degrees C after exposure to free radicals, whereas those from the dehydration-susceptible axes increased from 9 to 40 degrees C by the same free radical dose. The increased phase transition temperature was associated with a decrease in the phospholipid:sterol ratio, and an increase in the free fatty acid:phospholipid ratio. There was no significant change in total fatty acid saturation, which indicated that free radical treatment induced deesterification of membrane phospholipid, and not a change in fatty acid saturation. Similar compositional and structural changes have been previously observed in dehydration-injured soybean axes suggesting that dehydration may induce free radical injury to cellular membranes. Further, these membranes differ in their susceptibility to free radical injury, presumably reflecting compositional differences in the membrane since these membranes were exposed to free radicals in the absence of cytosol.