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A Novel Homogenous Assay for Topoisomerase II Action and Inhibition

Topoisomerase II is the only enzyme able to cleave and religate double‐stranded DNA; this makes it essential for many vital functions during normal cell growth. Increased expression of topoisomerase II is a common occurrence in neoplasia, and different topoisomerase II inhibitors have indeed been proven to be powerful anticancer drugs. For this reason, the topoisomerase II catalytic cycle has attracted strong interest, but only a few techniques contributing to studies in this field have emerged. All of the currently used conventional methods to elucidate the action and inhibition of topoisomerase II require separation steps and are therefore unsatisfactory in terms of sensitivity, speed, and throughput. Here, for the first time, we present an assay that works in homogenous solution. The assay is based on dual‐color fluorescence cross‐correlation spectroscopy (DC‐FCCS) and allows monitoring of topoisomerase II action and, especially, detection and discrimination of different topoisomerase II inhibitor classes. The effectiveness of our new assay was confirmed by measuring the effects of a catalytic inhibitor (novobiocin) and a topoisomerase poison (m‐AMSA) with bacteriophage T4 topoisomerase as a model system, thus showing the strategy to be easy, fast, and extremely sensitive. Further development of the DC‐FCCS‐based assay and subsequent application in high‐throughput drug screening of new anticancer drugs is proposed and discussed.