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.
