Identification and characterization of a high-affinity saturable binding protein for the carcinogen benzo(a)pyrene

A protein that binds the chemical carcinogen, benzo(a)pyrene (BP), in a high-affinity and saturable manner has been identified in cell extracts from the AKR-2B mouse embryo cell line. The BP bound to the carcinogen-binding protein (CBP) was exchangeable in a time- and temperature-dependent fashion and has a Kd of 1.8 X 10(-9) M. Competitive binding studies indicate that chemical carcinogens [3-methylcholanthrene, benz(a)anthracene, dibenz(a,c)anthracene] and other inducers of aryl hydrocarbon hydroxylase (5,6- and 7,8-benzoflavone) compete to varying degrees with BP for binding. Steroids, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and phenobarbital did not compete with BP for binding to the CBP, BP prevented the heat inactivation of CBP. Additional properties of CBP reveal it to have a Stokes' radius of 31 A, molecular weight of 61,000, frictional ratio of 1.2, an apparent isoelectric point of 5.2, and an S value of 4.8 in linear sucrose gradients. It was estimated that there are about 20,000 molecules of CBP per AKR-2B mouse embryo cell. The CBP was found in two nontransformed and one chemically transformed cell line; a fourth cell line A-431 (human vaginal carcinoma) had significantly reduced amounts of CBP.