A combined centrifugal, biochemical, and electron microscopic study of the cytoplasmic particulates present in 0.88 M sucrose homogenates of rat liver has been carried out. Size distribution analyses of particles containing pentose nucleic acid (PNA) and exhibiting several types of enzymatic activity revealed three major size groups within the range of particle radius between 10 and 500 mµ. A different array of biochemical properties was associated with each size group. The largest particles, with an average radius (assuming spherical shape) in the region of 220 to 260 mµ, contained all of the succinic dehydrogenase activity of the cytoplasmic extract, 29 per cent of the diphosphopyridine nucleotide (DPN)-cytochrome c reductase activity, and minor amounts of PNA and acid phosphatase activity. Cytologically, this group of particles was identified with the mitochondria. All of the uricase activity, 58 per cent of the acid phosphatase activity, and 26 per cent of the PNA was apparently associated with a second size group of particles (average radius 120 mµ) which were tentatively identified by electron microscopy with vesicular structures derived from the ergastoplasm of the intact cell. The third particle group demonstrated by centrifugation exhibited a major size distribution peak at 25 mµ and a second smaller peak at 55 mµ. Over 50 per cent of the total cytoplasmic PNA and DPN-cytochrome c reductase activity was associated with particles in this size group. Electron microscopy revealed a morphologically heterogeneous population of particles within this size range.

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