showed the inhibitory activity comparable to or greater than thiorphan (IC50 = 2.6 nM), a C-terminal carboxyl-containing inhibitor of enkephalinase. Thus compounds containing a C-terminal sulfogroup, instead of the C-terminal carboxyl group, were found to show a remarkably high level of inhibition of enkephalinase. The analgesic activity of 10b, (S)-10b, and (R)-10b was also evaluated by the phenylbenzoquinone