Met-enkephalin-Arg6-Phe7, present in high amounts in brain of rat, cattle and man, is an opioid agonist

Abstract
The enkephalins Met-enkephalin and Leu-enkephalin were first isolated from porcine brain by Hughes and co-workers. Recently isolated from bovine adrenals was another enkephalin with the structure Tyr-Gly-Gly-Phe-Met-Arg-Phe, or Met-enkephalin-Arg6-Phe7. This new heptapeptide is bound in human, rat and bovine striatum in concentrations comparable with or greater than that of Leu-enkephalin. This molecule should not be considered as a mere precursor of Met-enkephalin. A pharmacological study indicates that this naturally occurring enkephalin has similar properties to the 2 enkephalins characterized earlier.