IMPORT OF PHOSPHATIDYLETHANOLAMINE FOR THE ASSEMBLY OF RAT-BRAIN MITOCHONDRIAL-MEMBRANES

Mitochondria can synthesize phosphatidylethanolamine (PE) through phos phatidylserine decarboxylase (PS decarboxylase) activity or can import this lipid from the endoplasmic reticulum. In this work, we studied t he factors influencing the import of PE in brain mitochondria and its utilization for the assembly of mitochondrial membranes. Incubation of rat brain homogenate with [1-H-3]ethanolamine resulted in the synthes is and distribution of H-3-PE to subcellular fractions. Twenty-one per cent of labeled PE was recovered in purified mitochondria. The import of PE in mitochondria was studied in a reconstituted system made of mi crosomes (donor particles) and purified mitochondria (acceptor particl es). Ca+2 and nonspecific lipid transfer protein purified from liver t issue (nsL-TP) enhanced the translocation process. H-3-PE synthesized in membrane associated to mitochondria (MAM) could also translocate to mitochondria in the reconstituted system. Exposure of mitochondria to trinitrobenzensulfonic acid (TNBS) resulted in the reaction of more t han 60% of H-3-PE imported from endoplasmic reticulum and of about 25% of C-14-PE produced in mitochondria by decarboxylation of C-14-PS. Mo reover, the removal of the outer mitochondrial membrane by digitonin t reatment, resulted in the loss of H-3-PE, but not C-14-PE. These resul ts indicate that labeled PE imported in mitochondria is mainly localiz ed in the outer mitochondrial membrane, whereas PE produced by PS deca rboxylase activity is confined to the inner mitochondrial membrane. Ph ospholipase C hydrolyzed 25-30% of both PE radioactivity and mass of t he outer mitochondrial membrane indicating an asymmetrical distributio n of this lipid across the membrane.