Alkylation of DNA in rat tissues following administration of streptozotocin

Streptozotocin, an antibiotic widely used for induction of diabetes in experimental animals and for the treatment of pancreatic neoplasms, was shown to be a potent methylating agent reacting with DNA in vitro to form methylated purines. The reaction was similar in extent and relative proportions of methylation products to that produced by N-methyl-N-nitrosou-rea, the aglycone of streptozotocin. When streptozotocin was administered to rats by i.v. injection, DNA was methylated with the formation of 7-methylguanine, O⁶-methylguanine, 3-methyladenine, and 7-methyladenine in liver, kidney, intestine, and pancreas. In contrast to N-methyl-N-nitrosourea which produced approximately equal amounts of methylation in DNA of liver, brain, and kidney, streptozotocin caused virtually no methylation in brain DNA; but, both liver and kidney DNA were alkylated to a greater extent than with N-methyl-N-nitrosourea. This methylation of renal DNA may account for the ability of streptozotocin to induce renal tumors. Streptozotocin produced significant methylation of pancreatic DNA which, if concentrated in the β-cells, may account for their destruction. Pretreatment with nicotinamide reduced the extent of methylation of pancreatic DNA but did not affect the methylation in the liver or kidney. Methylation of β-cell DNA in the pancreas may lead to the initiation of tumors if the extent of alkylation is not so great that cell death occurs.