Structure-activity relations of fulicin, a peptide containing a D-amino acid residue

Structure-activity relations of fulicin (H-Phe-D-Asn-Glu-Phe-Val-NH2) isolated from the ganglia of a pulmonate mollusc Achatina fulica were investigated on the contraction of penis retractor muscle of this snail. The contraction-enhancing activity of fulicin was reduced to about 1/3000 when D-Asn2 was replaced by L-Asn, whereas substitution of the other D-amino acid for D-Asn2 showed no significant reduction of the activity. On the contrary, if a D-amino acid was introduced into any position other than position 2 the activity was reduced remarkably. The presence of Glu3 and five amino acid residues as a whole were also required for activity.