Purification and photoaffinity labelling of a rat cytosolic binding protein specific for 3-methylcholanthrene

Rat hepatic cytosolic proteins which sediment at 4-5 S on sucrose gradients exhibit high-affinity saturable binding for the carcinogen 3-methylcholanthrene. A rat liver protein of Stokes' radius 3 nm, Mr by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of 39,000 and with specific 3-methylcholanthrene-binding activity sedimenting at 4.5 S, has been purified 315-fold to apparent homogeneity by using affinity chromatography on a column of 1-hydroxy-3-methylcholanthrene coupled to epoxy-activated Sepharose 6B, in conjunction with two gel-filtration steps. The protein purified by this technique was shown to be associated with the observed specific 3-methylcholanthrene-binding activity by photoaffinity labelling with 1-oxo-3-methylcholanthrene.