the resulting analogue , 12 , had 1/4000th and 1/2400th the opiate receptor binding activity of leu-enkephalin when ( 3h ) [ d-ala2 , d-leu5 ] enkephalin and ( 3h ) naloxone , respectively , were used as tritiated ligands .
C0014294
leu-enkephalin
C0014298
enkephalin