Exchange of phospholipids between liver mitochondria and microsomes in vitro.

A 10% rat liver homogenate in 0.25 M sucrose-10-3 M ethylenediaminetetraacetate was fractionated into mitochondria, microsomes, and a supematant fraction. Liver mitochondria derived from rats injected with leucine-3H and with phosphate-32P or with glycerolJ4C were incubated with nonlabeled microsomes in the presence of supernatant or of sucrose-EDTA. Labeled microsomes were similarly exposed to nonlabeled mitochondria. 32P or glycerolJ4C was used to measure exchange of phospholipid; 3H was used to correct for contamination of mitochondria by microsomes. After 90 min at 37” 30 to 40% of the phospholipid in the particulate fractions had exchanged while at 0” 10% exchange took place. Phosphatidyl choline appeared to exchange to a greater extent than phosphatidyl ethanolamine, whereas cardiolipin of the mitochondrial fraction did not exchange. The extent of exchange was reduced 50 to 60% when sucrose-EDTA was substituted for the supernatant fraction as incubation medium.