Target size analysis of opioid receptors

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Abstract
Target size analysis of opioid receptor is complicated by the presence of multi-exponential inactivation curves. Irradiation of intact frozen tissue proved essential to eliminate such artifacts, due to indirect irradiation effects. Upon irradiation condition, opioid binding activity was inactivated in a single mono-exponential manner. Identical inactivation curves were obtained for .mu., .delta. and .kappa. binding activities in brain membranes from rat, guinea-pig and frog and in NG 108-15 cells: the molecular mass obtained was 98 .+-. 2 kDa. However, when opioid binding was assayed in the presence of Na+, Mg2+ and GTP, the molecular mass was found to be only 56 .+-. 4.4 kDa. We suggest that the opioid recognition site comprises a unit of 56 kDa and that in the absence of Na+, Mg2+ and GTP an additional membrane component of 40-44 kDa is necessary for high-affinity opioid binding.