Metabolism of dTTP in HeLa cell extracts, incorporation into DNA by isolated nudei and release of hypoxanthine from DNA by a hypoxanthine-DNA gtycosylase activity Bj0mar Myrnes,

dITP may be generated from dATP by a slow, nonenzymatic hydrolysis. While [H]dITP was degraded rapidly to [H]deoxyinosine by HeLa cell nuclear extracts, no net degradation of [H]dITP was observed in the presence of physiological concentrations of ATP, apparently because the extract contained deoxynucleoside diphosphate kinase activity that regenerated [H]dITP from [H]dIDP. Isolated HeLa cell nuclei, as well as partially purified DNA polymerase a, incorporated [H]dITP into DNA at 50-60* of the rate of [H]dGTP incorporation. No rapid release of the Incorporated radioactivity was observed. The molecular weight of nascent DNA containing dIMP residues, however, decreased slightly after prolonged incubation 1n the presence of EDTA, suggesting that a repair process is initiated in dIMP-containing chromatin. Furthermore, release of free [H]hypoxanthine from [H]dIMP-contain1ng DNA was detected after incubation with nuclear extracts in the presence of EDTA, suggesting the presence of hypoxanthine-DNA glycosylase activity in HeLa cell nuclei.