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

Abstract

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.