Enkephalins exhibit differential binding affinity to plasma proteins: Implications for plasma‐mediated effects on enkephalin bioactivity

Enkephalins are opioid peptides that act as neurotransmitters and modulators of cardiac function. Plasma enkephalin levels increase in response to ischemia and exercise. The level of bioactive peptides in blood is regulated at many levels, including inactivation by plasma or cell‐associated peptidases. Previous studies showed that Met‐enkephalin (ME) and Leu‐enkephalin (LE) bound to plasma proteins, and that binding to these proteins protected LE from proteolysis. We found that Met‐enkephalin‐Arg‐Phe (MEAP) and Met‐enkephalin‐Arg‐Gly‐Leu (MEAGL) also bound to human serum proteins; however, this binding was of significantly higher affinity that that of ME and LE. In addition, the enkephalin binding capacity of plasma was considerably greater than that of serum from the same donor, suggesting that enkephalins avidly bind to proteins that are present in plasma but not in serum. Finally, we observed a unique degradation pattern when MEAP was hydrolyzed by plasma carboxypeptidases, as compared to ME, LE and MEAGL. Together, our data suggest complex mechanisms by which enkephalin‐plasma interactions may regulate systemic enkephalin bioactivity.