THE INCORPORATION OF RADIOLABELED SULFUR FROM CAPTAN INTO PROTEIN AND ITS IMPACT ON A DNA-BINDING STUDY

Repeated administration of high doses of captan is known to produce tumours specifically in the duodenum of mice. Captan is not carcinogenic in the rat. In this study, DNA purified from the liver, stomach, duodenum and jejenum of mice dosed with S-35 radiolabelled captan was found to contain radioactivity equivalent to Covalent Binding Indices in the range 38-91; that from the bone marrow had a CBI of 2.8. The distribution of radioactivity between the various tissues did not reflect the target organ specificity of captan. Attempts to further purify the DNA samples using caesium chloride gradients resulted in partial separation of the radioactivity from the DNA suggesting that covalent binding to the DNA may not have occurred. A study of the chemical breakdown of captan showed that captan is unstable, producing a variety of potentially reactive species containing sulphur. Evidence was further obtained to show that the sulphur of captan is incorporated into endogenous amino acids and protein. Hepatic DNA from mice dosed with S-35 radiolabelled N-acetylcysteine, and two thiazolidine derivatives which are analogous to known metabolites of captan, was radiolabelled to a similar extent to that from captan treated mice. Furthermore, the DNA from each of these treatments had similar properties on caesium chloride gradients. It was concluded that the radioactivity associated with DNA in the captan DNA binding study was present in the low levels of protein which are always associated with purified DNA samples.