The Location of Different Synthetic Systems for Fatty Acids in Inner and Outer Mitochondrial Membranes from Rabbit Heart

The outer membranes have been separated from the inner membranes of rabbit heart mitochondria. The isolation of one unit from the other has been confirmed by assay of enzymes which have specific membranous locations and by electron microscopic examination. Fatty acid synthetic capacity and the products were studied in each of the subfractions. Two synthetic systems were present. A synthetic system de novo responsible for the complete synthesis of C14 to C18 fatty acids was contained in the inner membrane. The outer membrane possessed a chain elongation mechanism which added 1 or 2 acetate units to previously formed acyl compounds to make fatty acids 14 to 24 carbon atoms in chain length. The two systems differed in cofactor requirements and responsiveness to added NADH. It is suggested that the outer membrane synthetic system could function as an NADH shuttle by transferring cytoplasmic-reducing equivalents inside the mitochondrion as elongated fatty acids. The inner membrane system, which has a much larger capacity, could reoxidize intramitochondrial NADH in order to facilitate substrate level phosphorylation at times of respiratory chain inhibition.