Identification and quantification of human relaxin proteins by immunoaffinity-mass spectrometry

ABSTRACT The human relaxins belong to the Insulin/IGF/Relaxin superfamily of peptide hormones, and their physiological function is primarily associated with reproduction. In this study, we focused on a prostate tissue-specific relaxin RLN1 (REL1_HUMAN protein), and a broader tissue specificity RLN2 (REL2_HUMAN). Due to their structural similarity, REL1 and REL2 proteins were collectively named a ‘human relaxin protein’ in previous studies and were exclusively measured by immunoassays. We hypothesized that the highly selective and sensitive immunoaffinity-selected reaction monitoring (IA-SRM) assays could reveal the identity and concentration of REL1 and REL2 in biological samples and facilitate evaluation of these proteins for diagnostic applications. RT-PCR revealed the high levels of RLN1 and RLN2 transcripts in prostate and breast cancer cell lines. However, no endogenous prorelaxin-1 or mature REL1 were detected by IA-SRM in numerous biological samples. IA-SRM assay of REL2 revealed its undetectable levels (<9 pg/mL) in control female and male sera, relatively high levels of REL2 in maternal sera (median 331 pg/mL, 120 patients), and a biphasic expression of REL2 across the gestational weeks. IA-SRM assays discovered potential cross-reactivity and false-positive measurements for relaxin immunoassays. The developed IA-SRM assays will facilitate investigation of physiological and pathological roles of REL1 and REL2 peptide hormones.