Lysine residues located on the surface of human plasma high-density lipoprotein particles

Normal human plasma high-density lipoprotein (HDL) was reacted with trinitrobenzenesulfonate (TNBS). The time course of the reaction of intact HDL in phosphate buffer (pH 8.6) at 20 degrees C was biphasic, having a transition plateau. Both the first phase and the second phase reactions followed pseudo-first-order kinetics, and their rate constants at 20 degrees C were 4.6 X 10(-2) and 4.9 X 10(-3)/min, respectively. The activation energies of these reactions estimated from Arrhenius plots were 22.9 kcal/mol for the first phase and 20.9 kcal/mol for the second phase. The frequency factors, however, were markedly different from each other. On the other hand, the biphasic kinetic curves could not be observed when HDL was reacted with TNBS after delipidation, heat denaturation or Triton X-100 treatment. The rate constants for TNBS reactions of apolipoprotein HDL and heat-denatured HDL were essentially the same, 1.0 X 10(-2)/min, and those for HDL and apolipoprotein HDL in Triton X-100 were also the same, 3.57 X 10(-3)/min at 20 degrees C. Furthermore, fluorodinitrobenzene (FDNB), a typical penetrating chemical probe, reacted with intact HDL, indicating a nonbiphasic nature. These observations suggest that two reacting classes of lysine residues in intact HDL particles can be distinguished by the biphasic TNBS reaction, and that at the first phase of the reaction TNBS modifies selectively the accessible lysine residues exposed to surrounding aqueous environments. On this basis, intact HDL3 was reacted with TNBS at 20 degrees C for 80 min and it could be considered that at least 10 of 22 primary amino groups of apolipoprotein A-I and 10 of 18 lysine residues of apolipoprotein A-II were exposed on the surface of HDL3 particles.