Homogenates of suspensions were prepared aftergrowth in the absence and presence of chloramphenicol (500 μg./ml.). Sedimentation of organelles through gradients of aqueous sucrose solutions was studied by means of a zonal centrifuge. Mitochondria (marker enzyme, malate dehydrogenase) from organisms grown with chloramphenicol sediment more slowly (median sedimentation coefficient in water at 20° = 10,600 ) than those from normal cells (median sedimentation coefficient in water at 20° = 90,000 , which confirms that the former are of smaller volume than normal mitochondria. After high-speed zonal centrifugation both mitochondria (marker enzymes, NADH and NADPH-cytochrome oxidoreductases) attain similar equilibrium densities (median density = 1·21 to 1·22). The buoyant density (median = 1·23) of peroxisomes (marker enzyme, catalase) is not altered by the inclusion of chloramphenicol in the growth medium, but the proportion of non-sedimentable catalase is increased. Chloramphenicol-inhibited growth also leads to a change in the distribution of lysosomes (marker enzymes, acid -nitrophenolphosphatase, and N-acetyl-β(D)-glucosaminidase); a marked reduction in the population at = 1·15 to 1·20 is accompanied by the appearance at = 1·09 to 1·15 of a new class of organelles containing acid hydrolases.


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