Identification and analysis of exogenous peptides in human blood serum and plasma: Search for potential agents of interaction between the intestinal microbiota and the human body

ABSTRACT

Symbiotic interaction between the human body and its microbiota is an important issue of modern biomedicine and personalized medicine. However, little is known on molecular mechanisms of that relationship. Bearing in mind the ubiquitous participation of peptides in biomolecular interactions and regulatory processes we attempted direct search of blood peptides originated from microbial proteins. LC-MS/MS analysis was carried out of blood serum and plasma samples taken from 20 healthy donors on Q Exactive HF-X Hybrid Quadrupole-Orbitrap mass-spectrometer. Sample preparation was carried out based on our previously developed method of peptide desorption from the surface of major blood plasma proteins followed by standard chromatographic steps. Mascot and X! Tandem search engines were used for peptide identification. Human protein sequences were taken from UniProt Knowledgebase and sequences of human microbiota proteins-from NIH Human Microbiome Project (HMP). As a result, out of 13625 identified peptides 912 were unique fragments of microbial precursors, which is 6.69% of the total amount of detected bloodstream peptides. In 30 cases peptide identification was confirmed by mass-spectral study of individual synthetic samples. Absolute quantification by the mass-spectrometric method of multiple reaction monitoring (MRM) confirmed the presence of bacterial peptides in plasma and serum in the range of approximately 0.1 nMol/L to 1 mkMol/L, which is comparable to physiologically significant hormone concentrations in human blood in normal conditions. Analysis of the in silico obtained hydrolyzates of microbiotic proteins showed that significant number of the identified peptides are derived from the precursor proteins as a result of hydrolysis with trypsin, chymotrypsin and pepsin, the main proteases of the gastrointestinal system. 60% of the identified “microbial” peptides are derived from the intestine flora, about 20% - from oral microbiota and 20% fall on the remaining microbiotic communities. Most of the precursor proteins refer to intracellular, cytoplasmic proteins. The isolated fraction of peripheral blood mononuclear cells showed increase secretion of proinflammatory cytokines, colony stimulating factors and chemoattractants as the response to the addition of some of the identified microbiotic peptides. The data obtained serve as a basis for the ongoing study of the functional properties of microbiome derived peptides.