Quantification of ACE inhibiting peptides in human plasma using high performance liquid chromatography–mass spectrometry

An HPLC-MRM–MS method was developed for the quantification of 17 small ACE inhibiting (ACEI) peptides in plasma samples collected from human volunteers after the consumption of a peptide-enriched drink. The assay shows the high selectivity and sensitivity necessary to monitor small changes in the levels of the ACEI peptides after consumption of drinks developed to effect lowering of the blood pressure. Four different sample preparation methods were tested and evaluated. The final sample preparation method selected is simple and effective and consists mainly of the removal of proteins by acidification and heating, followed by a large volume injection. Additional sample preparation steps such as solid phase extraction and liquid/liquid partitioning were studied. Although they resulted in cleaner extracts, losses of specific peptides such as SAP were frequently seen. The isotope labeled form of one of the peptides to be quantified, [U13C]IPP, was used as an internal standard. The limit of detection of the assay is below 0.01 ng ml−1. The limit of quantification is between 0.05 and 0.2 ng ml−1, which is approximately 10% of the expected peptide concentration in plasma based on a normal diet. The intra- and inter-day relative standard deviations for all peptides have shown to be below 25% and the method has an accuracy of better than 75%. The long-term stability is good. At least 200 samples could be analysed before the system had to be cleaned. The assay has been successfully applied to blood samples collected from volunteers during a human trial.