beta-Endorphin: synthesis and analgesic activity of several analogs modified in positions 2 and 5.

The solid-phase syntheses of [Sar2]-, [Ala2]-, [D-Leu2]-, [D-Lys2]-beta-endorphins and [Pro5]-, [Leu5]-, [D-Leu5]-, [D-Ala2, D-Leu5]-beta-endorphins are described. The synthetic peptides were purified by chromatography on carboxymethylcellulose and partition chromatography on Sephadex G-50. They were characterized by partition chromatography on agarose, thin-layer chromatography, paper electrophoresis, and amino acid analyses of acid and enzymic hydrolysates. Bioassay of the synthetic analogs for analgesic activity by the tail-flick method showed the D-Leu2 analog to be 48% as potent as betah-endorphin while the Ala2, D-Lys2, Leu5, and [D-Ala2, D-Leu5] analogs were 8 to 17% as active. The Sar2, D-Leu5, and Pro5 analogs were less than 1% as potent.