Separation of mitochondria into two morphologically and biochemically distinct types

The mitochondrial pellet obtained from rat liver by the fractionation procedure of Hogeboom et al. (5) consist of a densely packed lower layer containing mitochondria, overlaid by a loosely packed fluffy layer. By repeated washing and resedimentation the fluffy layer material has been separated from the mitochondria of the dense pellet and from the microsomes. This isolated fluffy layer material consists almost exclusively of particles which are distinctly different from the mitochondria of the dense layer and the microsomes. They are somewhat smaller than the mitochondria of the dense layer, but their similarity to these mitochondria in shape, osmotic behavior, and succinoxidase activity, suggests that they are mitochondrial in nature, and we have therefore called them “small mitochondria”. However, in contrast to the large mitochondria, which have an extremely low PNA to proteinnitrogen ratio (less than 10 per cent of that of the microsomes), the small mitochondria have a ratio more than 80 per cent of that of the microsomes. Their alkaline phosphatase activity is approximately twice that of the large mitochondria or microsomes, but their phospholipid content is intermediate. Large and small mitochondria, apparently similar to those of rat liver, have been obtained from a number of tissues.