Combination Metabolomics Approach for Identifying Endogenous Substrates of Carnitine/Organic Cation Transporter OCTN1

Purpose Solute carrier SLC22A4 encodes the carnitine/organic cation transporter OCTN1 and is associated with inflammatory bowel disease, although little is known about how this gene is linked to pathogenesis. The aim of the present study was to identify endogenous substrates that are associated with gastrointestinal inflammation. Methods HEK293/OCTN1 and mock cells were incubated with colon extracts isolated from dextran sodium sulfate-induced colitis mice; the subsequent cell lysates were mixed with the amino group selective reagent 3-aminopyridyl- N -hydroxysuccinimidyl carbamate (APDS), to selectively label OCTN1 substrates. Precursor ion scanning against the fragment ion of APDS was then used to identify candidate OCTN1 substrates. Results Over 10,000 peaks were detected by precursor ion scanning; m/z 342 had a higher signal in HEK293/OCTN1 compared to mock cells. This peak was detected as a divalent ion that contained four APDS-derived fragments and was identified as spermine. Spermine concentration in peripheral blood mononuclear cells from octn1 gene knockout mice ( octn1 −/− ) was significantly lower than in wild-type mice. Lipopolysaccharide-induced gene expression of inflammatory cytokines in peritoneal macrophages from octn1 −/− mice was lower than in wild-type mice. Conclusions The combination metabolomics approach can provide a novel tool to identify endogenous substrates of OCTN1.