Abstract
The individual pretreatment of Sprague-Dawley rats with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 2,2',4,4',5,5'-hexachlorobiphenyl (HCB) has been previously shown to result in the "induction" of [3H]TCDD specific binding activity in hepatic tissue. In the present work, the coadministration of TCDD and HCB increased the concentration of hepatic proteins capable of binding [3H]TCDD specifically by at least 2-3-fold. This increase was shown not to be the result of activation, by HCB, of a form of the receptor having low affinity toward [3H]TCDD into a form with high affinity. Kinetic analysis of the time course of binding of [3H]TCDD to induced cytosol was consistent with the presence of an "inducible" binding protein in addition to the "constitutive" aryl hydrocarbon (Ah) receptor present in cytosol from untreated animals. The liganded ([3H]TCDD) form of the inducible binding component lost its ligand much faster than the liganded form of the constitutive Ah receptor at 37 degrees C; apparent first order rate constants for loss of [3H]TCDD were 0.55 min-1 and less than 0.0024 min-1, respectively. Conversely, the unliganded form of the induced binding component was slightly more stable (approximately 2-fold) toward thermal inactivation than the unbound constitutive Ah receptor. The [3H]TCDD-bound protein(s) in uninduced and induced cytosols behaved identically in a sucrose gradient; 8.7-8.9 S in the absence of salt, shifted to 5.5 S by 0.4 M KCl. They were also indistinguishable by gel permeation chromatography, and by photoaffinity labeling their TCDD-binding subunits, approximate molecular weights 105,000. These results show the hepatic TCDD-binding protein(s) induced upon pretreatment of Sprague-Dawley rats with TCDD/HCB to be kinetically distinct from the Ah receptor, but structurally very similar.
Highlights
The individual pretreatment of Sprague-Dawley toxicity is not observed in any single animal species; rats witheither 2,3,7,8-tetrachlorodibenzo-p-dioxin body weight loss, lymphoid involution, and/or immunotoxic
Kinetic analysis of the time course of binding of r3H]TCDDto induced cytosol was consistent mediated through an intracellular protein known as the aryl hydrocarbon (Ah) receptor, which binds TCDD and stereochemically related compounds with high affinity and specificity
Con- The constitutive levels of the Ah receptor may be estimated versely, the unliganded form of the induced binding by measuring the uptake of [3H]TCDD to specific binding component was slightly more stable (=2-fold) toward thermal inactivation than thuenbound constitutive Ah receptor.The [3H]TCDD-bound protein(s)inuninduced and induced cytosols behaved identically in a sucrose gradient; 8.7-8.9 S inthe absence of salt, shifted to5.5 S by 0.4M KCl. They were indistinguishable by gel permeation chromatography, and by photoaffinity labeling their TCDD-binding subunits, approximate molecular weights 105,000.These results show the hepatic TCDD-binding protein(s) induced upon pretreatment of Sprague-Dawley rats with TCDD/HCB to be kineticallydistinctfromthe Ah receptor, but structurally very similar
Summary
The individual pretreatment of Sprague-Dawley toxicity is not observed in any single animal species; rats witheither 2,3,7,8-tetrachlorodibenzo-p-dioxin body weight loss, lymphoid involution, and/or immunotoxic-. The coadministration ofTCDD tetrachlorodibenzo-p-dioxin (TCDD)’ in target tissues are and HCB increased the concentration of hepatic proteins capable of binding [3H]TCDD by at least 2-%fold This increase wasshown not to be the result of activation, byHCB, of a form of the receptor having low affinity toward[3H]TCDDinto a form with high affinity. 2,3,7,8-TCDDfirst associates with the soluble receptor protein; association is followed by transformation to a DNA-binding form and translocation into with the presence of an “inducible”binding protein in the nucleus [4,5,6,7,8] In this respect, the Ah receptor appears to addition to the “constitutive” aryl hydrocarbon (Ah) be a ligand-dependent regulatory protein possessing the abilreceptor present in cytosol from untreated animals. They were indistinguishable by gel permeation chromatography, and by photoaffinity labeling their TCDD-binding subunits, approximate molecular weights 105,000.These results show the hepatic TCDD-binding protein(s) induced upon pretreatment of Sprague-Dawley rats with TCDD/HCB to be kineticallydistinctfromthe Ah receptor, but structurally very similar
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