Description of a permeable eukaryotic cell system to study agents affecting DNA synthesis: demonstration that cytembena is a direct inhibitor of replicative DNA synthesis.

Cells were made permeable to DNTP's by a 15-minute treatment with 0.01 M Tris/HCl (pH 7.8), 0.25 M sucrose, 1 mM EDTA, 30 mM 2-mercaptoethanol, and 4 mM MgCl2 at 4 degrees C. These cells used exogenously supplied dNTP's to carry out semiconservative, replicative DNA synthesis. This system could be used to identify inhibitors of DNA synthesis and to determine whether chemotherapeutic agents affect precursor synthesis or whether they have a direct effect on the DNA replication complex. Thus DNA synthesis in the permeable cells was not inhibited by hydroxyurea or by the nucleoside analogue arabinofuranosyl cytosine. In contrast, the active form of the nucleoside analogue, arabinofuranosyl cytosine triphosphate, and daunorubicin promptly inhibited DNA synthesis in the permeable cells. Cytembena, sodium cis-beta-4-methoxybenzoyl-beta-bromoacrylate, also inhibited DNA synthesis in the permeable cells; this demonstrated that this agent functioned as a direct inhibitor of the DNA replication complex.