Submitted to: Lipids Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2005
Publication Date: 10/1/2005
Citation: Chen, L., Collins, X.H., Tabatabai, L.B., White, W.S. 2005. Use of a 13C tracer to investigate lutein as a ligand for plasma transthyretin in humans. Lipids Journal. 40:1013-1022.
Interpretive Summary: Lutein and zeaxanthine are dietary caratenoids present in plasma and accumulate in the macula. It has been observed that higher concentrations of these macular pigments in plasma are associated with lowered risk for the development of age-related macular degeneration (AMD). The proteins involved in the transport of the macular pigments have not been identified. This study describes experiments to determine whether transthyretin (TTR), a plasma protein, is the transporter for lutein. Methods were developed for the purification of transthyretin. Using plasma from control subjects and plasma from test subjects who were fed physiological amounts of lutein, it was determined that lutein was not associated with TTR, and that TTR may not be the physiological transporter for lutein into the macula.
Technical Abstract: The selective accumulation of lutein in the macula of the human retina is likely to be mediated by specific transport and/or binding proteins. Our objective was to determine whether transthyretin (TTR) is a plasma transport protein for lutein. We used a biosynthetic **13C-lutein tracer and gas chromatography-combustion interfaced-isotope ratio mass spectrometry (GC-C-IRMS) to gain the requisite sensitivity to detect the minute amounts of lutein expected as a physiological ligand for TTR. Subjects (n = 4) each ingested 1 mg of **13C-lutein daily for 3 days and donated blood 24 hours after the final dose. For 3 subjects, the plasma TTR-retinol-binding-protein (RBP) complex was partially purified by anion-exchange (DEAE) chromatography and then dissociated by hydrophobic-interaction chromatography to yield the TTR component. For a 4th subject, the initial DEAE purification step was omitted and total plasma TTR (RBP-bound and free) was isolated by hydrophobic interaction chromatography. In each case, the crude transthyretin fractions were then purified to homogeneity by RBP-Sepharose affinity chromatography. Pure TTR was extracted with chloroform and unlabeled lutein was added to the extract as a carrier. The mean **13C/**12C ratio (expressed in delta notation, delta**13C) of the lutein fraction isolated from the plasma TTR extracts of the 4 subjects was -30.53 ± 3.29 0/00. The delta**13C value of the unlabeled lutein carrier was -30.97 ± 0.27 0/00. Thus no **13C enrichment was detected in association with TTR. We conclude that lutein is not associated with TTR in human plasma after lutein is ingested in physiological amounts.