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Identification and Characterization of a High-Affinity Saturable Binding Protein for the Carcinogen Benzo(a)pyrene¹

Department of Cell Biology, Mayo Clinic and Foundation, Rochester, Minnesota 55901

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 K_d 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 Å, 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.

¹ This work was supported in part by National Cancer Institute Grant CA 22272, National Fraternal Order of the Eagles, and the Mayo Clinic and Foundation. Presented in part at the 72nd Annual Meeting of the American Association for Cancer Research, Washington, D. C., 1981 (55).

Received 4/13/82. Accepted 8/ 4/82.

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