Characterization of phospholipase A inhibition of stereospecific opiate binding and its reversal by bovine serum albumin

Opiate receptor binding is inhibited by phospholipase A from some sources, whereas the enzyme from other sources is inactive even at much higher concentrations. Evidence is presented that an active enzymatic site is required for inhibition and that the degree of inhibition seems to correlate with the extent of phospholipolysis. The inhibition is reversed almost completely by treatment with 0.5 to 1% bovine serum albumin, even up to 90% inhibition by phospholipase. As more enzyme is added or incubation time is increased, the extent of reversal diminishes. Based on our evidence, the most likely explanation of these results is that the inhibition of opiate binding activity by phospholipase A is due to the toxicity of the products of phospholipolysis and that bovine serum albumin reverses the inhibition by removing these products from the membranes, thereby restoring the active conformation of the receptors.