Isolation, identification and properties of a new thyroxine-binding protein--apolipoprotein A-1 from human blood serum]

A protein having a molecular mass of about 25 kWa was isolated by thyroxin (T4)-Sepharose affinity chromatography from human blood serum; its properties were found to be distinct from those of known T4-binding proteins. Using immunodiffusion, radioimmunoassay, lipid analysis, differential precipitation and electrophoresis, it was shown that the isolated protein is a component of high density lipoprotein (HDL) particles and represents an apolipoprotein A-1 (apoA-1). Using cholate-Sepharose chromatography apoA-1 was separated from the lipid moiety and contaminant proteins, and apoA-1 was effectively isolated directly from the blood serum. Apo-A-1-HDL and apoA-1 obtained by affinity chromatography as well as the HDL3 fraction isolated by a standard ultracentrifugation technique, all displayed a T4-binding activity, the affinities for the hormones being of the same order of magnitude. The T4 interaction with these preparations induced difference UV-absorption signals, altered the characteristics of apoA-1 intrinsic fluorescence without affecting the circular dichroism of the protein-hormone system. The binding of spin-labelled T4 to apo-1, apoA-1-HDL or HDI3 caused substantial changes in the shape of the ESR spectrum and an increase in the apparent rotational correlation time. The mobility of the radical fragment of spin-labelled T4 depended on the composition and properties of the protein preparation. The electron spectroscopy data suggest that the T4-HDL interaction occurs via specific mechanisms and that the molecular structures of the complexes formed thereby are not characteristic of other known T4-binding proteins.