Polycyclic Aromatic Hydrocarbon Binding Macromolecules. Identification, Characterization and Temperature Activation of a 4.5 S Binding Nucleoprotein

Purification to Homogeneity of the Major «4S» PAH Binding Protein from «Non Responsive » DBA/2N Mouse Liver by Affinity Chromatography

DBA/2N is a genetically non responsive inbred strain of mice in which administration of polycyclic aromatic hydrocarbons (PAHs) does not induce microsomal monooxygenase activity. DBA/2N mouse liver cytosol contains a polycyclic aromatic hydrocarbon-binding protein that sediments, in a sucrose gradient, at 4S (« 4S » PAH-BP). Its binding kinetic and physicochemical properties indicate that this protein is practically indistinguishable from the « 4S » PAH-BP identified and characterized in liver cytosol of rats and other PAH responsive rodents including C57 B1/6J mice. « 4S » PAH-BP was purified to homogeneity from DBA/2N mouse liver by ammonium sulfate fractionation of the cytosol, followed by Sephadex G-200 chromatography and, finally, affinity chromatography using 1-aminopyrene-Sepharose 6B. This procedure yielded about 50 μg of protein from 50–60 g of mouse liver, with a recovery of 18%. «4S » PAH-BP as a complex with3H-(benzo-a-pyrene) was more than 99% pure. A single band was seen on polyacrylamide gel electrophoresis under non denaturing conditions. H-BaP comigrated with the protein band.3H-BaP bound to the protein was displaced by PAHs with a specificity identical to that obtained using crude cytosol. On electrophoresis in SDS gels, the purified protein migrated as a single protein band with an apparent molecular weight of 40,000.

«4S» Polycyclic Aromatic Hydrocarbon Binding Protein. Its Role as a Benzo(a) Pyrene Cytosolic Carrier to the Microsomes of DBA/2N Mouse Liver

Rodent liver cytosol and other biological systems contain two proteins that bind polycyclic aromatic hydrocarbons (PAH) in a non covalent manner and that sediment at a different rate when centrifuged on sucrose gradient. The role of the smaller protein (« 4S » PAH-BP) was studied. When DBA/2N mouse liver homogenate was incubated with3H-BaP, most of the radioactivity was found in the microsomal subcellular fraction. The cytosol binding activity apparently decreased but reincubation of the cytosol with the radioactive ligand completely restored « 4S » PAH-BP activity. The microsomal uptake of3H-BaP can be studied in a reconstituted system in which microsomes are incubated with radioactive benzo(a)pyrene in the presence of crude cytosol. In these conditions the microsomal uptake rate of3H-BaP increased with the temperature and at 37 °C ten minutes were required to reach the plateau. When cytosol was substituted by HEDG buffer, the amount of radioactivity found in the microsomes decreased drastically. 0.2 μM was the benzo(a)pyrene concentration required to saturate the microsomes. When microsomes were incubated with ammonium sulfate cytosolic fractions or with homogeneously purified « 4S » PAH-BP, the3H-BaP uptake was restored and reached the maximum with 3 μg/ml of purified protein. The radioactive benzo(a)pyrene bound to microsomes was oxidated in the presence of NADPH regenerating system. The oxidated products were discharged from microsomes only when « 4S » PAH-BP was either present during the incubation or added at its end. Thus, this protein is able to transfer benzo(a)pyrene to the microsomal metabolization sites and to facilitate the release of oxidized products and, presumably, bind them.

«4 S» Polycyclic Aromatic Hydrocarbon Binding Protein: Further Characterization and Kinetic Properties

A protein that binds polycyclic aromatic hydrocarbons (PAHs) with high affinity and sediments in a sucrose gradient at 4 S has been described in rat liver cytosol. This « 4 S » PAH binding protein precipitates at a 40–60% ammonium sulfate saturation. This partial purification procedure allows assay of this protein by using purified3H-benzo(a)pyrene (3H-BaP) as radioactive ligand and dextran-coated charcoal as adsorbent for unreacted3H-BaP. The3H-BaP binding activity measured as a function of pH shows its maximum activity between pH 7.3 and 10.5. The « 4 S » PAH binding protein is stable up to 42 °C even in the absence of the ligand. At 65 °C the binding sites for3H-BaP are destroyed. The binding activity assayed as a function of protein concentration is linear between 0.4 and 2 mg/ml at 0 °C, whereas at 37 °C higher protein concentrations (4 mg/ml) can be reached. Exposure to guanidine-HCl (3 M) and urea (5 M) for 20 min at 4 °C inhibits the PAH binding completely to the « 4 S » protein. Quick dilution or dialysis does not restore the binding activity. The dissociation rate of the « 4 S » PAH binding protein measured in the presence of an excess of unlabeled ligand at 0 °C is biphasic and shows a two-step, first-order kinetic pattern. At 37 °C the dissociation rate is linear and faster, and is complete after 5 min of incubation. The association rate shows the same behavior: the binding is complete after 10 min at 0 °C, whereas at 37 °C the reaction is 10 times as fast. The dissociation equilibrium constants at 0 °C and 37 °C are respectively 2.45 × 10−9M and 1.09×10−9M. The high rates of association and dissociation of BaP to « 4 S » PAH binding protein were used to set up an assay to exchange radioactive3H-BaP with cold BaP.