In vivo kinetics of human apolipoprotein A-I variants in rabbits

Background. Generic variants of human apolipoprotein (apo) A-I, the major protein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plasma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apolipoprotein is a straightforward technique for determining its metabolism in vivo. In this study, we investigated the in vivo kinetics of several human apo A-I variants, which we had identified previously, in rabbits. Materials and methods. Apo A-I variants from heterozygous carriers of Lys→107→0, Lys-107→Met, Pro-3→Arg, Pro-4→Arg, Pro-165→Arg and Glu-198→Lys and the corresponding normal apo A-I were purified and then radioiodinated with ¹³¹I and ¹²⁵I. A kinetic study of apo A-I variants was performed in normolipidaemic rabbits after simultaneous injection of the two isotopes that had been incorporated into HDL. The fractional catabolic rate (FCR) was calculated from the radioactive decay curve. Results. Acidic mature (negatively charged) apo A-I variants caused by a single amino acid substitution (Lys-107→0, and Lys-107→Met) were catabolized faster (FCR, 1.931 ± 0.539 per day vs. 1.636 ± 0.460 per day, P ≤ 0.01 using the Wilcoxon signed-rank test) and basic mature (positively charged) apo A-I variants (Pro-3→Arg, Pro-4→Arg, Pro-165→Arg and Glu-198→Lys) were catabolized more slowly (FCR 1.470 ± 0.380 per day vs. 1.654 ± 0.430 per day, P ≤ 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an inverse linear relationship was observed between the deviation in the FCR of variant human apo A-I from that of normal human apo A-I and the number of electric charges that the apo A-I variant carried (r = -0.90, k = -0.188, P = 0.0003), as assessed by a linear regression analysis, suggesting that the electric charge of apo A-I variants may determine, at least in part, its in vivo kinetics in rabbits. Conclusions. Genetic variants of apo A-I with a single amino acid substitution show abnormal kinetics, and the electric charge of a apo A-I variant could contribute to determining its kinetics in vivo in this xenologous model.