Further characterization of the polycyclic aromatic hydrocarbon binding properties of the 4S protein

A 4-S protein which specifically binds [3H]benzo(a)pyrene and other polycyclic aromatic hydrocarbons has been investigated in the rat using a hydroxylapatite assay and sucrose gradient analysis. Although there was significant interanimal variation, the specific polycyclic aromatic hydrocarbon binding activity appeared to be highest in 4-week-old male rats and declined with age. The specific [3H]benzo(a)pyrene binding activity was induced after pretreatment with either phenobarbital or isosafrole as evidenced by a 72 and 61% increase, respectively, over untreated controls. No apparent increase in specific binding activity was observed after pretreatment of animals with 3-methylcholanthrene. Pretreatment with either phenobarbital or isosafrole also resulted in the appearance of a small, nonspecific, benzo(a)pyrene binding peak at the 8- to 9-S region in the sucrose density gradients. This 8-S peak was not seen in untreated control animals and represented low affinity, high capacity binding sites. In contrast to the 8-S protein, the 4-S binding protein had low affinity for polychlorinated aromatic compounds such as tetrachlorodibenzodioxin and tetrachlorodibenzofuran. The addition of a 200-fold excess of tetrachlorodibenzofuran to incubations did not displace [3H]benzo(a)pyrene from the 4-S protein. The addition of sodium molybdate to isolation buffers, known to stabilize certain hormone receptors, did not alter the sedimentation coefficient or the specific binding activity of the 4-S protein. These experiments indicate that the 4-S protein does not appear to be a subunit of the 8-S protein. We conclude that in the rat the 4-S protein is distinct from the 8-S protein and the 4-S species may regulate the polycyclic aromatic hydrocarbon-induced expression of aryl hydrocarbon hydroxylase activity.