Investigations into the mechanism of action of anti-tumour platinum compounds: time- and dose-dependent changes in the alkaline sucrose gradient sedimentation profiles of DNA from hamster cells treated with cis-platinum (II) diamminedichloride.

Under the conditions of low speed centrifugation used in this study, the proportion of radioactively labelled DNA from Chinese hamster V79-379A cells sedimenting to the 700S region of an alkaline sucrose gradient was increased in a dose-dependent manner by prior treatment of the cells for 2 h with cis-platinum (II) diamminedichloride [cis-Pt(II)]. This increase was at the expense of material sedimenting in the 400-650S region. This profile was not modified by a 2-h post-treatment incubation prior to centrifugation. 6 h after treatment, the DNA from treated cells sedimented in a narrow band at a position corresponding to 350S. 21 h after exposure to the drug, a dose-dependent restitution of the DNA species sedimenting in the 450-650S range was observed. These results combined with other data relating to platinum binding allow the following conclusions to be reached: (1) cis Pt(II) treatment does not lead to the rapid formation of single-stranded breaks or alkali-labile sites in cellular DNA. (2) The time-dependent changes in sedimentation rate of DNA from treated cells may reflect the transient appearance of gaps following endonuclease attack at platinum-bound sites in DNA. (3) The likely ratio of inter to intra-strand DNA-platinum interactions suggests that such endonuclease attack is primarily at platinum induced inter-strand DNA cross-links.