A concept to make low-abundance endogenous renin accessible to mass spectrometry: A multistep experimental design approach.

Renin is the rate-limiting step within the renin-angiotensin-aldosterone system, but the reliable quantification of human endogenous renin levels by liquid chromatography coupled with mass spectrometry remains challenging. The complex sample matrix triggering ion suppression and the detection of the low-abundance as well as the proteolytical-resistant renin make a hybrid approach using immunocapture coupled with LC-HRMS a promising method for investigation. Therefore, in-silico digestion and BLAST® experiments were conducted in order to identify the unique amino acid sequence for mass spectrometric detection. To enhance mass spectrometric response, impacting parameters within the denaturation, alkylation, and digestion experiments were identified and optimized by a multistep Design of Experiments process. The optimal denaturation buffer consisted of RapiGest® and urea, leading to a signature peptide intensity increase of 56% at 20 °C, whereas the optimal reducing agent improved intensity by 27%. The most effective generation of signature peptide I was achieved using a high trypsin concentration and a low incubation temperature enhancing digestion by 75%. The applicability of this hybrid approach was confirmed in human matrix and allowed for a fivefold reduction in total assay procedure time without limiting the reliable quantification if compared to a conventional digestion approach.