Ascorbate both Activates and Inactivates Bleomycin by Free Radical Generation7

ABSTRACT: The chemotherapeutic agent bleomycin (BLM) is activated by reducing agents to break isolated DNA. Paradoxically, these same reducing agents protect cellular DNA from BLM damage. To resolve this paradox, we have examined the reaction of FelIIBLM with DNA in the presence of ascorbate. As expected, ascorbate augments FeII'BLM-induced DNA damage. However, when ascorbate is added to FelIIBLM prior to exposure to DNA, a redox-inactive BLM is produced in a reaction that generates the ascorbyl radical. This reaction occurs in both ascorbate-supplemented buffer and unsupplemented plasma. In buffered solution, this reaction was found to be stoichiometric; for each mole of BLM present, 6.9 mol of ascorbate was oxidized and 4.7 mol of oxygen was consumed. Iron was found to serve only as a catalyst for the reaction. These data suggest that both activation of BLM and the generation of redox-inactive BLM occur via the same reaction and that BLM-induced DNA damage depends upon BLM reaching DNA prior to its interaction with reducing agents. Bleomycin (BLM), ' a glycopeptide antibiotic used in the treatment of several different neoplasms, is thought to exert its effects through cellular DNA degradation (Petering et al., 1990; Stubbe & Kozarich, 1987; Hecht, 1986). The current view is that FelIIBLM forms a complex with DNA; in the presence of reducing agents, the FelIIBLM is reduced to Fell-BLM which in turn combines with 0 2 and another reducing equivalent to form an "activated BLM". The activated BLM, H02--Fe111BLM, is highly oxidizing. It can abstract H ' fro