Structural essentials of Ser-1 in tethered peptide ligand of human thrombin receptor for phosphoinositide hydrolysis

In order to inspect the structural elements of Ser-1 in receptor activation by SFLLRNP (one-letter amino acid code), a ligand peptide tethered to the thrombin receptor, a series of analogs with such replacements as D-Ser, Ala, Thr, and Ac-Ser have been synthesized. These analogs were evaluated for their ability to hydrolyze the phosphoinositide in human neuroblastoma SH-EP cells. It was found that the α-amino group and L-configuration of Ser-1 are very important in the activation of receptors. N-Acetylation or deletion of Ser-1 completely eliminated the activity of SFLLRNP (a half-maximal effective concentration, EC₅₀ = 0.89 μM (1 M = 1 mol dm^-3)), and these modifications induced no antagonist activity. Incorporation of D-Ser also drastically diminished the activity, but retained about 50% activity of the maximal response by 100 μM SFLLRNP. The Ser/Ala substitution sustained 30% of the activity of SFLLRNP to elicit a full stimulation. The Ser/Thr substitution, however, enhanced the activity (20%) in spite of its decreased activity (60%) reported for platelet aggregation. These results indicated that the β-hydroxyl group of Ser-1 is important to receptor activation, but not essential. The effect of chemical modifications on the receptor activities of the tethered ligand is discussed with regard to the efficacy between phosphoinositide hydrolysis and biological activities.