Stereoselectivity of photoincorporation of [3H]6-AziP into β3 homomeric GABAA receptors. Five hundred micrograms of Sf9 cell membranes expressing His8-β3 homomeric GABAA receptors were photolabeled with 30 μM [3H]6-AziP [(3α,5β)-6-AziP] or its isomer [3H](3β,5β)-6-AziP. A, radioactivity in slices from SDS-PAGE of photolabeled β3 membranes. Inset, the 3α-isomer of 6-AziP labeled a 55-kDa protein, whereas the 3β-isomer did not. B, radioactivity in gel slices from SDS-PAGE of His8-β3 receptors purified on a nickel affinity column after photolabeling with either the 3α- or 3β-isomer of 6-AziP. The 3α-isomer labeled the 55-kDa protein (β3 subunit) twice as effectively did as the 3β-isomer.