The 27-kilodalton thyroxine (T4)-binding protein is human apolipoprotein A-I: identification of a 68-kilodalton high density lipoprotein that binds T4.

We have identified a previously described 66K T4-binding protein as a lipoprotein composed of two molecules of apolipoprotein A-I, five of cholesteryl esters, nine of phosphatidylcholine, and two of sphingomyelin. This 68.4K high density lipoprotein (HDL) corresponds to the minor approximately 67K HDL subfraction that we recently demonstrated as binding most of the HDL-associated T4. Since we have recently found that lipids inhibit the binding of T4 to apolipoprotein A-I, the very low lipid content (16 mol/mol) of this 68K HDL relative to that (greater than 100 mol/mol) of high mol wt HDL subfractions may account for the preferential binding of T4 to the low mol wt subfraction.