SUMMARY: Subcellular fractionation of the ciliate protozoon has been carried out using sucrose density gradients in zonal rotors. Fractions containing mitochondria, peroxisomes, ‘microsomes’ and organelles containing acid hydrolases have been characterized. Specific activities of acid phosphatase, DNase and -acetyl glucosaminidase in whole homogenates increase during the growth cycle, while those of mitochondrial enzyme systems (succinoxidase and 2-oxoglutarate oxidase) as well as those of NADH- and NADPH-cytochrome oxidoreductases decrease. Increased sedimentability of DNase and -acetylglucosaminidase is manifest in homogenates of cells grown for 6 days but the density distributions of the sedimentable portions of these enzymes and also that of acid phosphatase is notaltered. Marked changes in density distributions of lysosomal enzymes occur on starvation. Resolution of the heterogeneous lysosomal population into several discrete populations with different enzyme complements has been achieved. No significant alteration of mitochondrial density occurs in the stationary phase of growth or on starvation, whereas the median buoyant density of peroxisomes is increased under both these conditions.


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