Temperature-induced changes of spin-labelled radioactive lipids in isolated guinea pig liver microsomal membranes before and in mitochondrial membranes after their translocation

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Abstract
Lipids of isolated guinea pig liver microsomal membranes were labeled biosynthetically with isomeric doxyl stearic acid and temperature-induced changes of these membranes were studied by ESR. A noticeable discontinuity was detected at 10-12.degree. C with 12- or 16-doxyl stearic acid containing membrane lipids which was attributed to the spin-labeled lipid-microsomal membrane protein interactions since no such discontinuity was detected in liposomes prepared from total lipid extracts of microsomal membranes. When microsomal membranes containing radioactive isomeric spin-labeled lipids were incubated with unlabeled mitochondria, reisolated mitochondrial membranes contained translocated radioactive isomeric spin-labeled lipids. Temperature-induced changes in these membranes showed no discontinuity with either isomeric doxyl stearic acid derivative, establishing a difference in the environment of translocated lipids in the membrane donor compared with that in the membrane acceptor. Microsomal membranes recovered from translocation experiments showed the same behavior as the original membranes and exhibited the same discontinuity at 10-12.degree. C, establishing that the translocation incubation itself did not alter the spin-labeled lipid interaction within these membranes. Studies of the loss of paramagnetism of spin-labeled lipids in microsomal membranes before and in mitochondrial membranes after their translocation showed a significant difference and suggested that both the outer and the inner mitochondrial membranes might have been involved.