Translocation of spin-labelled radioactive lipids from isolated guinea pig liver microsomal to mitochondrial membranes

Translocation of membrane-bound labelled lipids from guinea pig liver microsomal to mitochondrial membranes was studied. When microsomal membranes containing known amounts and composition of spin-labelled radioactive lipids were incubated with unlabelled mitochondrial membranes, reisolated mitochondria contained spin- and radioactive-lipids in an amount which could not be accounted for by the microsomal contamination of reisolated mitochondria, establishing therefore the translocation of labelled lipids from microsomal to mitochondrial membranes. The effect of addition of crude 105000 X g supernatant on the translocation was studied. The translocation of labelled lipids in aged membranes was also described. The rate of loss of paramagnetism in microsomal and reisolated mitochondrial membranes was measured and found to be different, supporting the conclusion that the translocation of labelled lipids between membranes took place. Data from these studies suggested that the translocatndrial membranes probably occurred. Furthermore, our results suggest that the mechanism by which lipid translocation takes place does not necessarily involve soluble cytosol proteins. In view of experimental data, possible mechanisms for the translocation of lipids between biological membranes were discussed.