- BACKGROUND: The condensed state of the human sperm’s chromatin is essential for the compact structure of the spermatozoon head, which is important for the fertilization process. The enzymes DNA topoisomerases (topo I and topo II) are responsible for the topological structure of the chromatin in somatic cells. The activities and the characterization of topoisomerases in mature human sperm cells have not been previously investigated. METHODS: Sperm cells were purified from the semen of healthy donors by standard procedures and assays measuring the activities, protein size and sensitivity to inhibitors of topoisomerases were performed. RESULTS: Topo I and topo II DNA relaxation activities are present in nuclear extracts derived from human sperm. The sperm topo I activity is inhibited by camptothecin, similarly to the somatic enzyme. An 85 kDa sperm protein, compared with the 100 kDa somatic topo IB enzyme, reacted with anti-human topo I antibody. Sperm topo II lacks the DNA decatenation activity of the somatic enzyme and a 97 kDa protein, compared with the 170 kDa somatic topo IIa enzyme, was detected with anti-human topo II antibody. Sperm nuclear extracts contained inhibitors of somatic topo II decatenation activity. CONCLUSIONS: Topoisomerase I and II activities as well as topo I and topo II proteins are present in mature human sperm cells. These enzymes possess unique properties compared with their somatic counterparts.