Purified opioid mu-receptor is of a different molecular size than delta- and kappa-receptors.

Opioid mu-receptors were solubilized from rat brains with 0.5% Triton X-100 in the presence of 0.32 M sucrose and then purified with 6-succinyl morphine-Affi-Gel 102 column. Purified materials showed a major band with mol.wt. 58,000 Da and a minor band with mol.wt. 41,000 Da. Affinity cross linking with [3H]DAGO, a selective mu-opioid agonist, to the neural membranes and purified materials revealed that opioid mu-binding protein has a mol.wt. of 58,000 Da, a value totally different from that of delta- (mol.wt. 18,500 Da) and kappa- (mol.wt. 36,000 Da) binding protein.