Two allelic forms of the Ah receptor have been previously dentified by covalent labeling of the hepatic cytosol fractions of inbred strains of mice with the photoaffinity ligand 2-azido-3-[125l] iodo-7, 8-dibromodibenzo-p-dioxin and resolution of the labeled protein by denaturing gel electrophoresis: 1) a Mr 95,000 protein encoded by the Ahbi allele carried by the C57 and C58 family of mice, and 2) a Mr 1 04,000 protein encoded by the Ah#{176} 2allele present in other common inbred strains that are responsive to aromatic hydrocarbons(eg, C3H/He, BALB/cBy, and A). In this report, 125l-photoaffinity labeling is used to characterize two further munne variants and the strains that carry them: 1) the low affinity Ah receptor(Ahd allele) in strains that are nonrespon-sive to aromatic hydrocarbons and 2) a newly identified, high affinity variant (Ah’3) found in several strains recently derived from feral mice. The low affinity Ah receptor has been recently characterized by reversible ligand binding by Okey et a!.[Mo!. Pharmacol. 35: 823-830(1 989)], through the inclusion of sodium molybdate in the buffers during tissue preparation and Iigand incubation to stabilize the receptor. Examination of the Ah receptor in hepatic cytosol from 18 strains of mice carrying the Ahd allele, by preparation in molybdate and photoaffinity labeling, revealed that all strains express a Mr 1 04,000 protein. Tissue preparation in 20 mM sodium molybdate and subsequent dilution of the molybdate to--0.5 mi during ligand incubation was found to enhance photoaffinity labeling of the high and low affinity allelic forms of the Ah receptor. A new variant of the receptor(Ah#{176} 3) expressing a Mr 1 05,000 protein was detected in Mus molossinus, hortulanus, pahari, spretus, and carob but was absent from the strains of Mus muscu! us or domesticus that were examined. Allelic variants were also distinguishable by thermolability, ie, the half-life of specific ligand binding capacity upon incubation at 35#{176}. For the Ah1 allele (Mr 95,000) the t#{189}(thermostability) is 20-30 mm, for the Ah2 allele (Mr 104,000) the t#{189} S 3-6 mm, and for the Ah allele (Mr 105,000) the thermolability is intermediate.

The Ah receptor is a soluble protein detectable in a wide variety of tissues from vertebrate species that displays high affinity specific binding to certain planar aromatic compounds (ie, polycyclic aromatic hydrocarbons, typified by 3-methylcholanthrene, and halogenated aromatic hydrocarbons, typified by TCDD) and initiates the coordinate expression of a number of genes(i-3). The canonical and best understood response mediated by the receptor is the induction of cytochrome P1-450 (P45O1Ai) and associated monooxygenase activity(most often measured as AHH activity)(3). Several lines of evidence suggest that the ligand-receptor complex acts as a transcriptional activator by binding to specific“dioxin-responsive elements” with enhancer activity that are upstream from the first exon of the gene for cytochrome P1-450 (4, 5). The Ah receptor is presumed to be a member of the erb-A superfamily(6)(which includes the steroid hormone, vitamin D, retinoic acid, and