Rats were given radioactive L-leucine intravenously. At various times after injection, the livers were removed and separated into nuclear and cytoplasmic fractions by a nonaqueous technique. Glyceraldehyde-3-phosphate dehydrogenase, aldolase, and lactic dehydrogenase were isolated from each cell fraction by antibody precipitation followed by gel electrophoresis, and the specific radioactivities of the isolated enzymes were determined. In all three cases, the onset of labeling and the rate of incorporation were the same for the nuclear enzyme as for the corresponding enzyme from the cytoplasm. If we assume that equilibration of the enzymes between the cytoplasmic and nuclear pools occurs slowly relative to the labeling times employed, we may conclude that the labeled nuclear enzymes either were synthesized in the nucleus or moved into the nucleus from a cytoplasmic site of synthesis without first passing into the cytoplasmic pool of enzyme. Treatment with puromycin, an antibiotic which depresses incorporation into cytoplasmic proteins to a greater extent than into nuclear proteins, led to a situation in which the specific activities of the nuclear enzymes were several times as high as those of the corresponding cytoplasmic enzymes following a short period of incorporation. These data substantiate the assumption that equilibration between the cytoplasmic and nuclear enzyme pools occurs slowly and provide further evidence that the labeled nuclear enzymes do not arise from the cytoplasmic enzyme pool.

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