Simultaneous determination of tryptophan and its 31 catabolites in mouse tissues by polarity switching UHPLC-SRM-MS.

Tryptophan (TRP) and its catabolites have attracted a lot of attention because of their clinical significance to human health. Recently, microbiome-gut-brain axis was found to have links to many diseases based on the imbalance of TRP catabolism. By using ultra-high performance liquid chromatography coupled to electrospray ionization triple quadrupole mass spectrometry, we present a rapid, robust and comprehensive method to determine 31 TRP catabolites covering three major pathways - kynurenic, serotonergic and bacterial degradation - within 5 min. Polarity switching was employed to analyze catabolites in both ionization modes simultaneously for greatly improved analytical throughput. The intra-day and inter-day precision were 0.5-15.8% and 1.5-16.7%, respectively. Accuracy was between 75.8 and 126.9%. The developed method was applied to study the tissue level of TRP catabolites in the liver, ileum, ileal contents, brain and plasma samples from 8 mice, and clear differences in the distribution of TRP catabolites were observed in different tissues. Ratios of key catabolites to TRP were used to evaluate the activities of specific enzyme and pathway in respective tissues. This method has potential in high throughput analysis of TRP catabolites in biological matrices, which can facilitate understanding the influence of TRP catabolites on microbiome-gut-brain axis and on human health.