Chloramphenicol-mediated DNA damage and its possible role in the inhibitory effects of chloramphenicol on DNA synthesis

Studies were undertaken to examine the interaction of chloramphenicol and some of its analogs with DNA in an effort to elucidate the mechanism by which high levels of the drug cause inhibition of DNA synthesis. Chloramphenicol at concentrations of 1 mM and above was found to mediate the degradation of double-stranded DNA in the presence of copper and a reducing agent. Similarly, the l-threo steroisomer of chloramphenicol was equally potent at causing strand scissions in DNA. Nitroso-chloramphenicol, which inhibits DNA synthesis at much lower concentrations, also causes DNA damage at levels 100-fold lower than chloramphenicol. In contrast, thiamphenicol, which has a methyl-sulfonyl group in place of the nitro group at the para position, neither affects DNA synthesis nor causes DNA degradation under the conditions tested. The good correlation between the inhibition of DNA synthesis and the generation of strand-scissions by this series of analogs suggests that damage to DNA may be responsible for the inhibition of DNA synthesis seen with chloramphenicol, rather than an interaction of chloramphenicol with DNA polymerase. This proposal is further substantiated by studies with a DNA polymerase in a cell-free system. There was no inhibition of DNA synthesis when any of the analogs were added directly to the polymerase reaction mixture. However, a significant, time-dependent reduction in DNA synthesis was seen when the DNA template used for the polymerase assay was preincubated with chloramphenicol or nitroso-chloramphenicol prior to its use in the assay, again suggesting damage to DNA as the mechanism involved.