Biological activity of TRH thionalogue and its diastereoisomers

Binding affinity and TSH-releasing activity of [Prot3] TRH analogue (L-pyroglutamyl-L-histidyl-L-proline thioamide), TRH and their LDL and LLD diastereoisomers were compared in rats. Binding affinity and dose-related increase of TSH secretion were similar for both [Prot3] TRH and TRH. Moreover, similar effect of both peptides on sleeping time and motoric activity of rat was found. Substitution of L- for D-amino acids of TRH and [Prot3] TRH decreased the binding affinity and TSH-releasing activity as well. It was concluded that [Prot3] TRH analogue is as active as native TRH.