Background: Signal peptides may be novel biomarkers in cardiovascular diseases. Methods: We developed a novel immunoassay to the signal peptide of preproCNP (CNPsp) and used this to document circulating venous concentrations of CNPsp in normal healthy volunteers (n = 109), regional plasma CNPsp concentrations in patients undergoing clinically indicated catheterisation (n = 24) and temporal CNPsp concentrations in patients with ST-elevation myocardial infarction (STEMI) <4 h after symptom onset (n = 8). The structure/sequence of circulating CNPsp was confirmed by tandem mass spectrometry (MS/MS). Results: In normal human plasma, CNPsp was detectable at levels higher than NT-proCNP (74 +/- 17 vs. 20 +/- 5.5 pmol/L). There was no correlation between NTproCNP and CNPsp, but plasma concentrations of sibling signal peptides - CNPsp and BNPsp - were strongly correlated (r = 0.532, P < 0.001). In patients undergoing catheterisation, there were significant arterio-venous step-ups in CNPsp concentrations across the heart (P < 0.01) and kidney (P < 0.01). Arterial concentrations of CNPsp significantly correlated with heart rate (r = 0.446, P < 0.05). In STEMI patients, plasma concentrations of CNPsp showed a biphasic elevation pattern between 6 and 12 h after symptom onset, with 12 h values significantly elevated (similar to 3-fold) compared with levels at presentation (P < 0.05). MS/MS verified circulating CNPsp to be preproCNP(14-23) and preproCNP(16-23) peptides. Conclusions: This is the first report of a circulating preproCNP derived signal peptide. Given the clear cardiac and renal secretion profiles of CNPsp and its response in STEMI patients, further studies on potential biological functions and biomarker applications of CNPsp in cardiovascular disease are warranted. (C) 2014 Elsevier Inc. All rights reserved.