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

Background Genetic variants of human apolipoprotein (apo) A-I, the major pr otein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plas ma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apol ipoprotein 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-10 7-->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 radioiodin ated with I-131 and I-125. A kinetic study of apo A-I variants was performe d in normolipidaemic rabbits after simultaneous injection of the two isotop es 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 sin gle amino acid substitution (Lys-107-->0, and Lys-107-->Met) were cataboliz ed faster (FCR, 1.931 +/- 0.539 per day vs. 1.636 +/- 0.460 per day, P less than or equal to 0.01 using the Wilcoxon signed-rank rest) and basic matur e (positively charged) apo A-I variants (Pro-3-->Arg, Pro-5-->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 less than or equal to 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an in verse 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 e lectric 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 substituti on show abnormal kinetics, and the electric charge of a apo A-I variant cou ld contribute to determining its kinetics in vivo in this xenologous model.