Analysis of the interaction between tryptophan-related compounds and ATP-binding cassette transporter G2 (ABCG2) using targeted metabolomics

•LC–MS/MS analysis of tryptophan metabolome in plasma and milk shows ABCG2 interaction.•Abcg2 mediates milk secretion of metabolites of the kynurenine pathway in mice.•Bovine ABCG2 Y581S polymorphism increases kynurenine concentration in milk.•Kynurenine and kynurenic and anthranilic acid are in vitro substrates of murine Abcg2.•The bovine ABCG2 Y581S polymorphism affects the in vitro transport of kynurenine. ATP-binding cassette transporter G2 (ABCG2) is involved in the secretion of several compounds in milk. The in vitro and in vivo interactions between tryptophan-related compounds and ABCG2 were investigated. The tryptophan metabolome was determined by liquid chromatography-tandem mass spectrometry in milk and plasma from wild-type and Abcg2−/− mice as well as dairy cows carrying the ABCG2 Y581S polymorphism (Y/S) and noncarrier animals (Y/Y). The milk-to-plasma ratios of tryptophan, kynurenic acid, kynurenine, anthranilic acid, and xanthurenic acid were higher in wild-type mice than in Abcg2−/− mice. The ratio was 2-fold higher in Y/S than in Y/Y cows for kynurenine. In vitro transport assays confirmed that some of these compounds were in vitro substrates of the transporter and validated the differences observed between the two variants of the bovine protein. These findings show that the secretion of metabolites belonging to the kynurenine pathway into milk is mediated by ABCG2.