Physical association of oestrogens and other steroids with DNA

4C-Labelled mestranol, 3-O-methyl oestrone and cholesterol-5 alpha,6 alpha-oxide have been prepared. Along with the natural oestrogens, E1 and E2, and other steroids, these compounds have been used to determine the extent of their physical association with DNA. Analysis of binding both by equilibrium solubilization and by caesium chloride density gradient centrifugation showed the same relative order of binding: mestranol greater than 3-O-methyl oestrone greater than oestrone greater than cholic acid greater than cholesterol-5 alpha,6 alpha-oxide greater than progesterone, testosterone greater than oestradiol. DNA from Micrococcus lysodeikticus showed a higher affinity for cholic acid than did calf thymus DNA, while pretreatment of the latter with proteinase K somewhat reduced the level of physical binding of oestrone.