Incorporation and turnover of tritium in neonatal mice and their mothers after feeding tritiated thymidine during pregnancy

After feeding mice tritiated thymidine during the entire pregnancy, a study was performed of the incorporation of tritium into DNA, protein and supernatant fraction, the DNA turnover times and the fraction of low turnover in various organs of neonates and their mothers. A total of 2 - 2.5% of the total activity in the neonatal mouse was incorporated into DNA, while a slightly larger amount entered proteins and lipids together. Only about 2% of the supernatant fraction but, dependent on the organ, from 10 to 50% of the macromolecules in the neonate have a long life span and about 20-30% of the DNA may be metabolically inert. Thus it may be calculated that 100-150 ppm of the ingested thymidine contribute to molecules of long life span. In adults about 1% of the supernatant and about 5-10% of the macromolecules have a long life span. The rest of the activity, (2000-4000 ppm in neonates, 40,000-60,000 in adults) has a half life of less than 10 days. (U.K.)