Selenite: a good inhibitor of rat-liver DNA methylase

DNA methylase from rat liver was partially purified through a DEAE sephacel column and characterized in an in vitro assay with respect to time, protein, DNA and S-adenosylmethionine curves. The Km for S-adenosylmethionine was 2.5 microM. Sodium selenium inhibited the methylation of DNA in a dose dependent fashion when added to the assay. It was also demonstrated that selenite non-competitively inhibits rat-liver DNA methylase with a Ki of 6.7 microM. Dithiothreitol had no effect on selenite inhibition and increasing amounts of DNA did not alter the inhibition. However, increasing amounts of protein overcame the inhibition, suggesting that selenite is reacting with the DNA methylase protein. DNA methylase isolated from selenite treated animals had only 43% of the activity as enzyme from control rats. It appears that selenite is a good inhibitor of DNA methylase.