Inhibition of PM-2 DNA degradation by a human serum protein

A unique DNA-binding protein was detected that inhibited DNA degradation induced by bleomycin and was decreased in sera of cancer patients. The protein from normal human serum was purified to homogeneity by ammonium sulfate precipitation and DEAE-cellulose and DNA-cellulose column chromatography. Two-dimensional isoelectric focusing gel electrophoresis revealed a single protein spot with a molecular weight of 64,000 and a pI at pH 5.9. The NH2 terminus was lysine, and the ratio of acidic to basic residues was 1.2. DNA binding was demonstrated by column chromatography, agarose gel electrophoresis, fluorescence quenching, and circular dichroism. The inhibitory activity was abolished by treatment with Pronase but not by RNase or DNase I. FeCl2 caused a partial loss of inhibitory activity. The inhibition of DNA degradation was more effective for breakage induced by bleomycin than neocarzinostatin, macromomycin, or DNase I. Evidence from DNA-binding studies suggests the inhibition is due to binding of the protein to sites on DNA preferred by bleomycin. Thus, the protein could be useful for studies on the mechanism of action of bleomycin and other antitumor agents, the cytotoxic effects of which are due primarily to damage of cellular DNA. The protein was decreased significantly in sera of cancer patients, and its potential use as a diagnostic tool for neoplasias is being investigated further.