Further enhancment of analgesic activity: enkephalin analogs with terminal guanidino group.

There is a great body of evidence that a basic amino-terminal is essential for the opioid activity of enkephalins, Tyr-Gly-Gly-Phe-Met and -Leu [ 1 I, and their analogs. Removal of the amino-group of Tyr results in a practically inactive peptide [2], as does its acetylation [3]. Introduction of two methylgroups into the terminal amino-moiety brings about a substantial loss in the biological activity [4], while mono-methylation decreases the potency in the mouse vas deferens preparation but enhances activity in the guinea-pig ileum [S]. It was tempting to speculate that interaction between the peptide N-terminus and its counterpart on the receptor (presumably a carboxyl-group) might be favourably influenced by replacing the terminal amino-group with a guanidino function. The resonance stabilized guanidinium cation has the capacity to interact.with some bidentate structure in the receptor site establishing hydrogen bridges in addition to the electrostatic interaction. One could anticipate that the improved binding properties of the peptide N-terminus would result in enhanced biological activity. It has also to be taken into account, however, that this replacement leads to a distal shift of the positive charge (~1.2 A). Such modification at the e-amino group of the lysine side chain in certain peptides, i.e., conversion of lysine into homoarginine residue, makes the lysine bond resistant to trypsin [6]. Thus trypsin seems to be unable to combine simultaneously with both the carbonyl-group and the positive charge of the basic residue when the distance between these is greater than occurs in a lysine or arginine residue. In this context one may recall that papain, which is less specific than trypsin, can hydrolyse homoarginine