Phosphorylation of enkephalins enhances their proteolytic stability.

Pharmacological action of enkephalins as opioid peptides is limited because of their rapid degradation by endoproteases. A novel approach is used in this study to prolong the life of those peptides. Phosphorylation of N-terminal tyrosine residue is found to have a profound influence in improving the stability of [Met]enkephalin and [Leu]enkephalin against the action of aminopeptidase M. Whereas, breakdown of [Met]enkephalin and [Leu]enkephalin is essentially complete in less than one min when incubated at 37 degree C with purified aminopeptidase M (EC3.4.11.2; substrate:enzyme = 1:0.1) in Tris buffer (pH 7.02), the corresponding phospho analogs are still detected 60 min after start of incubation. The rate of disappearance of phospho-[Met]enkephalin and phospho-[Leu]enkephalin follows first-order kinetics with half-lives of 7.3 and 8.3 min, respectively.