3H-thymidine mediated and delayed nuclear labeling in frog lens epithelium. Evidence for reutilization of DNA precursors

After intraperitoneal injection of ³H-thymidine into frogs, substantial levels of radioactivity can be measured in the serum and aqueous humor for at least a week. Transplantation of non-radioactive lenses into ³H-thymidine injected frogs and, conversely, the reinjection of radioactive serum and aqueous humor into non-radioactive recipients produced nuclear labeling in autoradiographs of lenses for periods up to 3 days after injection.Gel-filtration and paper chromatography of serum from ³H- and ¹⁴C-thymidine injected frogs indicate that the labeling of dividing lens cells is mediated by free thymidine in the first 3 h after injection, and later by reutilization of a high molecular weight radioactive substance appearing in the serum from 6 h on. Demethylation of thymidine may occur in the frog.A review of the recent literature shows that reutilization of precursors derived from initially labeled DNA also occurs in mammals. Caution must be exercised, therefore, in interpreting autoradiographic studies of cell division, movement, and transformations since delayed labeling after injection of ³H-TdR appears to be a widespread phenomenon.