Mechanism of action of deoxyribonuclease II from human lymphoblasts

Deoxyribonuclease II has been purified through five fractionation steps from the human lymphoblast cell line K562. Isolation included DEAE-cellulose and heparin-agarose chromatography followed by fractionation on Mono-S, Mono-Q and Superose-12 FPLC columns. In an extension of previous studies, deoxyribonuclease II was found to introduce a much higher proportion of single-strand nicks relative to double-strand breaks into supercoiled DNA than has been reported for linear DNA. Application of DNA sequencing techniques has further revealed a unique resistance of 3' termini to hydrolysis by this enzyme. Deoxyribonuclease II cleaves at every available site along the duplexed portion of a paired oligonucleotide substrate with the exception of the last four nucleotides. Consistent with previous results, this deoxyribonuclease II is active at low pH in the absence of Mg2+ and is not inhibited by EDTA, but complete inhibition is observed with 100 microM Fe3+. Likewise we confirmed the presence of 3'-phosphoryl termini on the DNA cleavage products since they failed to function as primers for DNA synthesis catalyzed by Escherichia coli DNA polymerase I.