CHARACTERISTICS OF BINDING TYROXINE WITH FRACTIONS OF HIGH DENSITY LIPOPROTEINS AND APOLIPOPROTEIN A-I

The aim of the study was to study the binding of various fractions of human blood plasma lipoproteins (VLDL, LDL, HDL2 and HDL3) with thyroxine (T4). Chromatography on Sepharose CL-2B showed that a significant portion of the labeled125I-thyroxine eluted in the volume of the output of the HDL fraction, which may indicate the possibility of formation of HDL-T4 complexes. The process was reversible, since the introduction of an excessive amount (10-6-10-7 M) of unlabeled T4 into the system resulted in the displacement of125I-T4 from complexes with HDL, which indicates a «protein - ligand» type interaction. The effect of environmental conditions on the formation of complexes of T4 with HDL is investigated. An increase in temperature to 40 °C resulted in a more than 50 % decrease in the release of radioactivity with a fraction of HDL. Complexation decreased at 4 °C, however, the complexes showed the greatest stability. The main physicochemical characteristics of the interaction of T4 with subfractions of HDL2 and HDL3 were obtained by quenching tryptophan fluorescence. The complexation constants of HDL with T4 were of the order of 106-107 M-1. The greatest affinity for T4 was found for particles of subfraction of HDL3. It was established that the main protein component of HDL ─ apolipoprotein A-I takes part in complexation with T4, on which has one binding center.