Synthesis and opioid activities of stereoisomers and other D-amino acid analogs of methionine-enkephalin

A series of D-amino acid-substituted analogs of the opiate peptide, methionine⁵-enkephalin, were synthesized by solid-phase methods and tested for their abilities to inhibit electrically-evoked contractions of mouse vasa deferentia and to compete with tritiated enkephalin for opiate receptors on particulate fractions isolated from homogenates of rat brain. [D-Ala²]-enkephalin and [D-Ala²]-enkephalin amide were found to be the most potent peptides in both assay systems, being about 1000% active in the vas deferens bioassay and 120% and 150% active, respectively, in the stereospecific binding test relative to methionine⁵-enkephalin itself. In comparison, [D-Met⁵]-, [D-Tyr¹]-, [D-Leu²]-, [D-Phe²]-, [D-Ala³]-, and [D-Phe⁴]-enkephalin had not more than 10% activity. The stabilization of the β-bend conformation of methionine⁵-enkephalin by the substitution of D-alanine in position 2 of the peptide chain may contribute to the high activities of the [D-Ala²]-analogs.