Contribution of the Superoxide Anion-Hydroxyl Radical Pathway to the Cleavage of DNA by Bleomycin

Abstract

The bleomycins (BLM) are a group of glycopeptide antibiotics which exhibit c linically useful antineoplastic activity and are isolated from Streptomyces verticillus ^(1,2). They are effective against squamous cell carcinoma, Hodgkin’s disease, and malignant lymphoma and are particularly valuable since they show no immunosuppression⁽²⁾. The mode of action has attracted a great deal of attention but the molecular mechanism of action is by no means clear. BLM shows interference with nucleic acid metabolism as evidenced by (i) inhibition of DNA synthesis⁽³⁾, (ii) interference with mitosis⁽²⁾, and (iii) DNA breakage⁽⁴⁾. Recently BLM has been shown to release nucleosomes from chromatin⁽⁵⁾. In addition there is interference with enzyme action, e.g., inhibition of ligase action⁽⁶⁾, but stimulation of nuclease activity⁽⁷⁾. BLM also causes liberation of DNA from DNA-membrane complexesi⁽⁸⁾. Thus considerable biochemical evidence suggests that the principal cell target is DNA which is degraded by BLM both in vivo and in vitro ^(2,4,9-11). DNA cleavage is enhanced in the presence of reducing agents such as 2-mercaptoethanol, dithiothreitol, or ascorbate and by the addition of Fe(II) and requires oxygen^(2,1l-13). The DNA cleavage is inhibited by Cu, Zn, Co, Mg, and EDTA⁽¹⁴⁾. These observations led to the suggestion of a direct role of trace metal ions, particularly of Fe, in the mode of action of the antibiotic^(12,13) which would be in accord with a general role suggested for Fe in tissue disorders and cancer control⁽¹⁵⁾. Clinical grade BLM was shown to contain 0.02 mole% Fe by atomic absorption⁽¹⁵⁾.