Purification and identification of endogenous anti-opioid substances from bovine brain

Two fractions with high potency for reversing the inhibitory effect of Met-enkephalin on the electrically induced contractility of guinea-pig ileum have been purified from bovine brain extract. Unexpectedly, one isolated peptide was identified as [Val5]-angiotensin I and the other fraction was [Val5]-angiotensin II, as judged by chromatographic comparisons on HPLC and amino acid analysis. Since angiotensins did not affect opioid binding to brain membrane, we consider that angiotensins may act as physiological antagonists to the opioid system in the brain, as well as in the guinea-pig ileum.