1. The metabolism of chromosomal proteins has been studied in the pancreas, liver, and kidney of adult mice (a) by measuring the rates of glycine-N15 incorporation into histones and residual chromosome fractions, and (b) by measuring the extent to which N15, once incorporated into chromosomal proteins, is retained.

2. The uptake of isotopic nitrogen by these nuclear constituents was compared with that of protein fractions prepared from the cytoplasm by differential centrifugation in sucrose solutions. One such fraction, which comprises the bulk of the ribosenucleoprotein of the cell sediments as a pellet on high speed centrifugation. The supernatant remaining after this centrifugation is a fraction which, in the pancreas, is rich in the secretory enzymes synthesized by the cell.

3. A comparison of the rates of glycine-N15 uptake shows that cytoplasmic ribonucleoprotein is the most active of the protein fractions analyzed. In the pancreas it meets the conditions required of a precursor for the secretory enzymes of the supernate.

4. In all tissues considered the rates of glycine-N15 uptake into histone and residual chromosome fractions are lower, that for histone being the lowest of any of the protein components considered and that for residual protein approximating the over-all rate for cytoplasmic protein.

5. The effects of feeding and fasting upon glycine-N15 incorporation have been studied. In the pancreas, feeding causes a sharp increase in N15 uptake by the mixed tissue proteins and by the nucleoprotein and supernatant protein of the cytoplasm. There is a parallel increase in N15 uptake by the chromosomal constituents—histone and residual protein.

6. A parallelism between N15 uptake in cytoplasmic and chromosomal proteins is also observed in the liver and kidney when over-all protein metabolism is altered by feeding and fasting.

7. The responsiveness of the histones and residual proteins to changes in the environment has also been demonstrated in N15 retention experiments. The loss of isotope once incorporated into chromosomal proteins is much more rapid in fed than in fasted animals.

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