Design, recombinant expression and convenient A‐chain N‐terminal europium‐labelling of a fully active human relaxin‐3 analogue

Relaxin-3 (also known as INSL7) is a recently identified neuropeptide belonging to the insulin/relaxin superfamily. It plays a putative role in the regulation of food intake, in the stress response and in reproduction by activating the G-protein-coupled receptor, RXFP3. In a previous study, we prepared 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)/Eu3+-labelled human relaxin-3 as a tracer for the study of ligand–receptor interactions, which necessitated a complicated site-specific labelling strategy because human relaxin-3 contains four primary amine moieties, all of which react with the primary amine-specific modification reagent. To simplify the labelling procedure, in the present study we created an easily labelled, recombinant analogue of human relaxin-3 with only one primary amine moiety at the A-chain N-terminus. The analogue retained full activity and could be easily labelled by various functional probes at the A-chain N-terminus. The DOTA/Eu3+-labelled analogue retained high binding affinity for its cognate receptor, RXFP3, and thus represents a useful, nonradioactive and stable tracer for studying the interaction of RXFP3 with various natural or synthetic ligands. This new analogue is also a suitable template for the design of other relaxin-3 analogues that can be easily labelled with the DOTA/Eu3+ moiety and used to study binding activity and interactions with various RXFP3 analogues in the future.