A study of the incorporation of glycine-N15 by chicken red cells in vitro has shown that:

1. There is no detectable nitrogen turnover in the histone or desoxyribonucleic acid of erythrocytes or reticulocytes.

2. Hemoglobin synthesis in the nucleated reticulocyte proceeds at 2 to 3 times the rate observed in the mature erythrocyte.

3. The uptake of glycine-N15 by heme is 9 to 14 times the corresponding uptake into hemoglobin, and 12 to 20 times the calculated uptake into globin.

4. Maturation of the red cell results in a decline in the rate of synthesis of both heme and globin, but the deceleration is much more marked in globin. synthesis.

5. No significant differences could be detected in the low N15 incorporations of nuclear and cytoplasmic hemoglobins.

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