DNA Interaction with 57Co-Bleomycin

Tumor-diagnostic 57Co-bleomycin is a mixture of two isomers: types I and II. Interaction between these and DNA was studied by fluorescence spectrometry and thermal denaturation. The fluorescence study indicated that cobalt chelation resulted in a remarkable increase in the apparent DNA-bleomycin association constant and a slight increase in bleomy-cin-DNA binding; a remarkable difference was observed between the two isomers. In the thermal denaturation study, the difference of DNA binding behavior was also observed. The tumor affinity of these isomers was slightly different, and type I isomer showed higher tumor affinity than type II. These results indicate that cobalt chelation to bleomycin enhances DNA-bleomycin binding and its DNA binding stability, and these mechanisms, although not fully understood, appear to underly the difference in tumor affinity of cobalt-bleomycin isomers.

A thermal denaturation study of chromatin and nuclease-produced chromatin fragments

Calf thymus chromatin and nuclease-produced chromatin fragments have been examined by thermal denaturation measurements. Native chromatin gave a series of distinct melting transitions at 64, 73, 79, and 85 °C in 0.25 mM EDTA pH 8. Treatments such as dialysis, mechanical shearing, or sulfhydryl oxidation of histone H3 carried out on native chromatin significantly altered the derivative melting profiles by blurring the distinct transitions and shifting the highest melting transition to a lower temperature. Derivative melting profiles for electrophoretically purified chromatin fragments, monomer through hexamer, all resembled that obtained from dialyzed chromatin. These results suggest that higher order structures exist in chromatin that are easily disrupted. Since the products of micrococcal nuclease (EC 3.1.4.7) digestion of the altered chromatins did not exhibit any major electrophoretic differences from those obtained from nuclei, then most likely the primary arrangements of histones along the DNA are the main determinant for cleavage sites.