Characterization of a Digitonin‐Solubilized Bovine Brain H3 Histamine Receptor Coupled to a Guanine Nucleotide‐Binding Protein

: The H3 receptor is a high‐affinity histamine receptor that inhibits release of several neurotransmitters, including histamine. We have characterized H3 receptor binding in bovine brain and developed conditions for its solubilization. Particulate [3H]histamine binding showed an apparently single class of sites (KD= 4.6 nM; Bmax= 78 fmol/mg of protein). Of the detergents tested, digitonin at a detergent/protein ratio of 1:1 (wt/wt) yielded the greatest amount of solubilized receptors, typically 15–30% of particulate binding. Neither equilibrium binding of [3H]histamine to receptors (KD= 6.1 nM; Bmax= 92 fmol/mg of protein) nor the inhibitor profile was substantially altered by digitonin solubilization. However, solubilization did increase the rate of [3H]histamine association with and dissociation from the receptor. Size‐exclusion chromatography indicated an apparent molecular weight of 220,000 for the solubilized receptor, and peak binding from this column retained its guanine nucleotide sensitivity. These last two observations are consistent with the solubilized receptor occurring in complex with a guanine nucleotide‐binding protein.