Effect of alkylation with streptozotocin on the secondary structure of DNA.

S1 nuclease hydrolysis and hydroxyapatite chromatography were used to study the effect of the alkylating antibiotic, streptozotocin, on the secondary structure of DNA. Native calf thymus DNA was alkylated in vitro with increasing concentrations of streptozotocin and subjected to S1 nuclease hydrolysis. An increasing degree of DNA degradation was seen, suggesting a destabilization of the secondary structure. Indirect evidence, deduced from alkaline hydrolysis, effect of NaCl on S1 nuclease hydrolysis, and hydroxyapatite chromatographic analysis of alkylated DNA, suggested a significant alkylation of DNA phosphates in addition to DNA bases. Nicotinamide has been reported to alter the cytotoxic and carcinogenic effects of streptozotocin. Our experiments indicate that in the presence of nicotinamide, streptozotocin causes the formation of a greater proportion of alkylated bases in relation to alkyl phosphotriesters. This may have significance in relation to the differential cytotoxicity of streptozotocin in the absence and presence of nicotinamide.