Phospholipase A inhibition of opiate receptor binding can be reversed by albumin

Abstract

OPIATE receptors in animal and human brain have been shown to be tightly associated with cell membranes^1–3. Our laboratory has reported the solubilisation of an etorphine–receptor complex⁴, but to date it has not been possible to obtain an active receptor in soluble form. Nevertheless, it has been possible to study many biochemical characteristics of the membrane-bound receptor. Its sensitivity to sulphhydryl reagents and a number of other protein reagents^5,6 as well as to proteolytic enzymes^1,7 suggests the participation of protein(s) in opiate binding. The evidence is less clear with respect to a function for phospholipids. Opiate binding activity of cell membrane preparations from rat brain has been shown to be exquisitely sensitive to phospholipase A of Vipera russelli venom (in the ng ml⁻¹ range)⁸, but very insensitive to phospholipase A present in the venom of Crotalus adamenteus¹ or that derived from pig pancreas (Lin & Simon, unpublished). Phospholipase C is inhibitory only in very high concentrations^1,8. The great sensitivity of opiate receptors to the Russell's viper enzyme and to ionic and non-ionic detergents^4,7 points to a possible role for phospholipids in the binding process. We report here that inhibition of opiate binding by phospholipase A can be reversed almost completely by incubation of the enzyme-treated membranes with bovine serum albumin (BSA).