Bistranded DNA damage produced by the enediyne antitumor antibiotic calicheamicin (CAL) was examined in plasmid DNA and in a model site for CAL-mediated DNA damage containing the sequence AGGA.TCCT. CAL was observed to produce bistranded DNA damage to the virtual exclusion of single-strand (SS) lesions. Without expression of drug-induced abasic sites as strand breaks, CAL activated by 10 mM glutathione produced equal numbers of DS and SS breaks in plasmid DNA. However, cleavage of drug-induced abasic sites with hydrazine and putrescine resulted in a dramatic increase in the appearance of bistranded damage, with ratios of DS to SS breaks of approximately 6:1 and approximately 24:1, respectively. A similar bias for DS lesions was noted in CAL-mediated damage in the AGGA.TCCT model sequence, in which evidence for a 4'-hydroxylated abasic site was observed on the AGGA strand. These model DNA systems revealed three important features of the bistranded damage produced by CAL. First, the major lesion produced by CAL is a bistranded lesion consisting of an abasic site on one strand and a direct strand break on the other. Second, 3'-phosphoglycolate residues resulting from CAL-induced C4'-hydrogen abstraction at the C of AGGATC were restricted to bistranded lesions. This unusual behavior of the C4'-chemistry of both CAL and the related neocarzinostatin (NCS) is consistent with an intermediate in the partitioning that is dependent on bistranded lesions. Finally, 4-hydroxythiophenol activated CAL to produce a ratio of DS to SS lesions of approximately 10:1, as opposed to the approximately 1:30 ratio noted previously for NCS.(ABSTRACT TRUNCATED AT 250 WORDS)