107 identifying the nature and function of these

Info icon This preview shows pages 569–571. Sign up to view the full content.

View Full Document Right Arrow Icon
107 Identifying the nature and function of these factors may provide important clues about the mechanisms of antitumor activity, drug resistance, or repair. Study of protein-DNA-drug interactions is an essential feature of the bioinorganic chem- istry of platinum chemotherapeutic agents. 4. Mapping the major adducts of cis- and trans-DDP on DNA; sequence specificity As we have seen, the antitumor activity of cisplatin is most likely the result of its DNA-binding properties. But what are the adducts? The human genome has more than a billion nucleotides. Does platinum recognize any special re- gions of the DNA or any particular sequences? In other words, is binding simply random or is there at least a regioselectivity? In this section, we discuss the best strategies for answering these questions, strategies that evolved in pursuit of learning how cis-DDP binds to DNA. We also illustrate their power in elucidat- ing the DNA-binding properties of other metal complexes of interest to bio- inorganic chemists. a. Early Strategic Approaches The first experiments to imply the se- quence preferences of cis-DDP binding to DNA employed synthetic poly- mers. 108 ,109 Specifically, the buoyant density of poly(dG)' poly(dC), poly(dG' dC), and their cis-DDP adducts was studied in the analytical ultracentrifuge. The greatest shift in buoyant density was seen for the platinum adducts of poly(dG) .poly(dC), from which it was concluded that platinum forms an intra- strand crosslink between two neighboring guanosine nucleosides on the same strand. This interpretation was suggested by the known preference of metal ions, and especially platinum, for binding at the N7 position on the guanine base (Figure 9.9), information available from model studies of metal-nucleobase chemistry. Although other interpretations of the buoyant-density shift were pos- sible, especially since the amount of platinum bound was not quantitated, the conclusion proved to be correct, as confirmed by later investigations. Interest- ingly, trans-DDP did not selectively increase the buoyant density of poly(dG)' poly(dC). Following these initial experiments, the regioselectivity of cis-DDP binding was investigated by studying the inhibition of enzymatic digestion of platinated DNA. For example, the platinum complex inhibits the cleavage of DNA by restriction enzymes that recognize specific sequences and cut both strands of the double helix. ItO The resulting fragments are readily identified on electrophoresis gels. One such restriction enzyme is Bam HI. As shown by the arrows in Scheme (9.11), Bam HI cleaves a six-bp palindromic sequence at the phosphodiester bonds between two guanosine nucleosides. Formation of an intrastrand crosslink between the two adjacent guanosine nucleosides inhibits digestion by the en- zyme. Another method, termed exonuclease mapping, involves digestion of the
Image of page 569

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
552 9 I METALS IN MEDICINE strands of duplex DNA from its 3' _ends. 111 ,112 When the enzyme encounters a bound platinum atom, it is unable to proceed further. Analysis of the digestion
Image of page 570
Image of page 571
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern