Discovery and Identification of New Amino Acid-Conjugated Bile Acids by Polarity-Switching Multiple Reaction Monitoring Mass Spectrometry

Bile acids (BAs) are a class of vital gut microbiota-host cometabolites, and they play an important role in maintaining gut microbiota-host metabolic homeostasis. Very recently, a new mechanism of BA anabolic metabolism mediated by gut microbiota (BA-amino acid conjugation) has been revealed, which provides a perspective for the research on BA metabolism and gut metabolome. In this study, we established a polarity-switching multiple reaction monitoring mass spectrometry method to mine amino acid-conjugated bile acids (AA-BAs) derived from host-gut microbiota cometabolism. Using the developed method, we discovered and identified 14 types of amino acid conjugations with host BAs for the first time including serine, valine, threonine, cysteine, asparagine, aspartic acid, lysine, glutamine, glutamic acid, methionine, histidine, arginine, isoleucine, and tryptophan conjugations in mouse and human feces. Moreover, we found that the AA-BA metabolism was closely related to the physiological state of the host. Our findings greatly expand the chemical diversity of gut microbiota-derived AA-BAs and lay a foundation for the study of their specific mechanisms and biological functions.