Radiation Inactivation Studies of the Benzodiazepine/γ‐Aminobutyric Acid/Chloride Ionophore Receptor Complex

Abstract: Radiation inactivation was used to estimate the molecular weight of the benzodiazepine (BZ), γ‐amino‐butyric acid (GABA), and associated chloride ionophore (picrotoxinin/barbiturate) binding sites in frozen membranes prepared from rat forebrain. The target size of the BZ recognition site (as defined by the binding of the agonists [³H]diazepam and [³H]flunitrazepam, the antagonists [³H]Ro 15–1788 and [³H]CGS 8216, and the inverse agonist [³H]ethyl‐β‐carboline‐3‐carboxylate) averaged 51,000 × 2,000 daltons. The presence or absence of GABA during irradiation had no effect on the target size of the BZ recognition site. The apparent molecular weight of the GABA binding site labelled with [³H]muscimol was identical to the BZ receptor when determined under identical assay conditions. However the target size of the picrotoxinin/barbiturate binding site labelled with the cage convulsant [³⁵S]t‐butylbicyclophosphorothionate was about threefold larger (138,000 daltons). The effects of lyophilization on BZ receptor binding activity and target size analysis were also determined. A decrease in the number of BZ binding sites (B_max) was observed in the nonirradiated, lyophilized membranes compared with frozen membranes. Lyophilization of membranes prior to irradiation at – 135°C or 30°C resulted in a 53 and 151% increase, respectively, in the molecular weight (target size) estimates of the BZ recognition site when compared with frozen membrane preparations. Two enzymes were also added to the membrane preparations for subsequent target size analysis. In lyophilized preparations irradiated at 30°C, the target size for (i‐galactosidase was also increased 71% when compared with frozen membrane preparations. In contrast, the target size for glucose‐6‐phosphate dehydrogenase was not altered by lyophilization. Thus, in some cases lyophilization may lead to an increase in protein target size, possibly due to aggregation of protein subunits. We conclude that in frozen membrane preparations the functional target size of the BZ and GABA binding sites reflects the subunit molecular weights for the BZ/GABA/chloride ionophore receptor complex.