Asymmetric distribution of exogenous thymidine among pyrimidine isostichs suggests compartmentalization of replicative DNA synthesis during regeneration in rat liver.

The distribution of radioactivity among pyrimidine isostichs (or isoplyths) of DNA from 24-h regenerating rat liver was studied with [3H]Thd, [14C]orotate or with inorganic 32Pi. Expression of incorporated radioactivity as log10% of total radioactivity recovered for each of the 11 pyrimidine isostichs detected showed that radioactivity from [3H]Thd was asymmetrically distributed among the isostichs, i.e., 3H radioactivity failed to access regions of DNA yielding lower molecular weight pyrimidine isostichs as efficiently as it accessed regions yielding higher molecular weight pyrimidine isostichs. The thymine (T) content of isostichs exceeded that of cytosine (C), i.e., T/C ratios for the first 10 isostichs averaged 1.43 +/- 0.08 and 1.28 +/- 0.05, depending on the method of analysis; furthermore, the T/C ratio for isostich 1 was significantly higher than ratios for isostichs 2 through 10. Asymmetric distributions of [3H]Thd radioactivity also were seen at 18 or 30 h post-partial hepatectomy. Thus, radioactivity from [3H]Thd, a DNA precursor from the salvage pathway, failed to efficiently access lower molecular weight isostichs despite thymine enrichment, suggesting that thymine moieties were supplied from additional sources. Radioactivity from [14C]orotate accessed lower molecular weight pyrimidine tracts more efficiently than [3H]Thd, but less efficiently than it accessed higher molecular weight isostichs, resulting in an asymmetric distribution of 14C radioactivity. This result suggested that appreciable quantities of thymine and cytosine moieties utilized for DNA synthesis were supplied de novo, but other sources also were utilized. Radioactivity from 32Pi, a de novo precursor, was distributed symmetrically, i.e., the slope among lower molecular weight isostichs increased enough that it was indistinguishable from slopes for intermediate and higher molecular weight isostichs. Since 32P radioactivity among lower molecular weight isostichs reflects appreciable contributions of de novo phosphate moieties from both pyrimidine- and purine-containing deoxynucleoside triphosphates, opportunities for observing contributions of 32P radioactivity from pathways other than the de novo pathways appeared to lie beyond limits of detectability. The distribution of radioactivity from labeled DNA precursors among lower molecular weight pyrimidine tracts (a) indicate that thymine moieties are contributed by both salvage and de novo pathways; (b) support the possibility that cytosine moieties also are contributed by both pathways; and (c) support the 'replitase' concept for channeling dNTPs to replicating forks.