Cleavage of endorphins to des-tyr endorphins by homogeneous bovine brain aminopeptidase

Attention has recently focused on the possible biological roles of peptides related to β-1ipotropin (β-LPH)^1,2. In particular, observations suggest that certain peptide fragments derivable from β-LPH, including γ-endorphin (β-LPH_61-77) and α-endorphin (β-LPH_61-76), may be involved in various modes of behavioural adaptation^2,3. Structure-activity studies on behavioural actions of γ-endorphin, an opioid peptide, revealed that the omission of N-terminal tyrosine resulted in a non-opioid peptide, des-Tyr-γ-endorphin, which exhibited greater behavioural activity than γ-endorphin³. Subsequent biochemical studies provided evidence for the association with brain synaptic plasma membranes of peptidase activities which could produce γ- and α-endorphins and their des-Tyr derivatives from exogenous β -endorphin⁴. This and earlier observations^4ndash;6 suggest that regulatory physiological events are focused on the N-terminal tyrosine residue; enzymatic reactions exerted at this residue are thus of great interest. We recently purified to homogeneity an aminopeptidase from bovine brain which catalyses the hydrolysis of Met⁵- or Leu⁵-enkephalin by sequential release of amino acids from the amino terminus of the enkephalin molecule⁷. The ability of this enzyme, purified from bovine⁷, monkey⁸ or rat⁹ brain, to use endorphins as substrates has not been previously examined. We now present quantitative studies which show that homogeneous bovine brain enkephalin aminopeptidase catalyses the hydrolysis of the N-terminal tyrosine from γ-, α- and β -endorphins with substantial turnover numbers and yields only the des-Tyr derivatives. These results suggest that this enzyme may be involved in regulating the concentrations of opioid- and opioid-derived biologically active peptides in the brain.