This family includes transthyretin that is a thyroid hormone-binding protein that transports thyroxine from the bloodstream to the brain. However, most of the sequences listed in this family do not bind thyroid hormones. They are actually enzymes of the purine catabolism that catalyse the conversion of 5-hydroxyisourate (HIU) to OHCU [ PUBMED:16098976 … expand

This family includes transthyretin that is a thyroid hormone-binding protein that transports thyroxine from the bloodstream to the brain. However, most of the sequences listed in this family do not bind thyroid hormones. They are actually enzymes of the purine catabolism that catalyse the conversion of 5-hydroxyisourate (HIU) to OHCU [ PUBMED:16098976 PUBMED:16462750 ]. HIU hydrolysis is the original function of the family and is conserved from bacteria to mammals; transthyretins arose by gene duplications in the vertebrate lineage [ PUBMED:16952372 PUBMED:8428915 ]. HIUases are distinguished in the alignment from the conserved C-terminal YRGS sequence.

Transthyretin (formerly prealbumin) is one of 3 thyroid hormone-binding proteins found in the blood of vertebrates [ PUBMED:1833190 ]. It is produced in the liver and circulates in the bloodstream, where it binds retinol and thyroxine (T4) [ PUBMED:4054629 PUBMED:24422526 PUBMED:33001386 ]. It differs from the other 2 hormone-binding proteins (T4-binding globulin and albumin) in 3 distinct ways: (1) the gene is expressed at a high rate in the brain choroid plexus; (2) it is enriched in cerebrospinal fluid; and (3) no genetically caused absence has been observed, suggesting an essential role in brain function, distinct from that played in the bloodstream [ PUBMED:1833190 ]. The protein consists of around 130 amino acids, which assemble as a homotetramer that contains an internal channel in which T4 is bound. Within this complex, T4 appears to be transported across the blood-brain barrier, where, in the choroid plexus, the hormone stimulates further synthesis of transthyretin. The protein then diffuses back into the bloodstream, where it binds T4 for transport back to the brain [ PUBMED:1833190 ].