Abstract
Matrix inclusions in mitochondria of prenatal brown fat and liver are rich in lipids, and it has been suggested that these should mainly be phospholipids. This proposition has been investigated. Pellets of glutaraldehyde-fixed mitochondria were extracted with excess hexane or chloroform-methanol (2:1 v/v); in controls buffer was substituted for organic solvent. Following extraction the pellets were postfixed in OsO4. Matrix granules in buffer-treated mitochondria (controls) were strongly osmiophilic and were bleached by H2O2 treatment. In hexane-extracted mitochondria the inclusions were less intensely osmiophilic, but they showed the same response to H2O2 as did the controls. Extraction with chloroform-methanol resulted in empty spaces in place of the matrix granules and H2O2 treatment did not produce significant changes. The fact that the inclusions were not extracted by hexane suggests that they are not composed mainly of neutral lipids. The solubility and osmiophilia of the material in the matrix granules are compatible with the reactions expected of phospholipids, but the evidence does not exclude the possibility that other classes of organic compounds might also be involved.