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Transthyretins isolated from different species bind hydrophobic compounds and are often obtained in a yellow form. Such a transthyretin from chicken serum was purified by chromatography using Sepharose-coupled human retinol-binding protein. The yellow chromophore was extracted with methanol and purified by reverse phase HPLC followed by normal-phase chromatography on a nitrile column. Ultraviolet-visible absorbance and mass spectrometry identified the yellow compound as iutein, i.e. xanthophyll, (all-trans)-~,e-carotene-3,3"-diol, estimated to constitute 10-30% of associated eolourless compounds. These components are different from the yellow component isolated from human transthyretin and establish that carotenoidderived pigments can be associated with transthyretins.

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Transthyretin (TTR) in plasma is associated with yellow compounds. Their properties differ, and in the chicken protein a major yellow compound has recently been identified as a carotenoid, lutein, also called xanthophyll. We now show that the major yellow component extracted from human TTR has properties like a pterin derivative, 7,8-dihydropterin-6-carboxyaldehyde (2-amino-4-hydroxy-6-formyl-7,8-dihydropteridine). The human TTR derivative has chromatographic and spectral properties identical to a yellow photochemical degradation product of biopterin and a spectrum like that of the pterin aldehyde.

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