Apolipoprotein A-I and A-II Kinetic Parameters as Assessed by Endogenous Labeling With [H3]Leucine in Middle-Aged and Elderly Men and Women

The purpose of our study was to investigate high density lipoprotein (HDL) apolipoprotein (apo) A-I and apoA-II kinetics in a state of constant feeding after a primed-constant infusion of [5,5,5-H3]L-leucine in 32 normolipidemic older men and postmenopausal women (aged 41 to 79 years). ApoA-I and apoA-II were isolated from plasma HDL, and enrichment was determined by gas chromatography/mass spectrometry. The fractional secretion rate was obtained by using a monoexponential equation calculated with the SAAM II program (Department of Bioengineering, University of Washington, Seattle). Mean HDL cholesterol (HDL-C) and total triglyceride levels were 23% higher and 27% lower, respectively, in women than in men. Mean plasma apoA-I levels were 10% greater in women than in men, whereas mean apoA-II levels were similar. HDL size, estimated by gradient-sizing gels and by the HDL-C/apoA-I1apoA-II ratio, was significantly higher in women than in men. Mean apoA-I secretion rates (SRs) were similar in men and women (12.2863.64 versus 11.9662.92 mg/kg per day), whereas there was a trend toward a lower (213%) apoA-I fractional catabolic rate (FCR) in women compared with men (0.19960.037 versus 0.22560.062 pools per day, P50.11). Mean apoA-II SRs (2.2160.57 versus 2.2760.91 mg/kg per day) and FCRs (0.17960.034 versus 0.18160.068 pools per day) were similar in men and women. For the group as a whole, there was an inverse association between the HDL-C/apoA-I1apoA-II ratio and apoA-I FCR and between the ratio and triglyceride levels. Plasma levels of apoA-I and apoA-II were correlated with their respective SRs but not FCRs. These data suggest a major role for apoA-I and apoA-II SRs in regulating the plasma levels of these proteins, whereas apoA-I FCR might be an important factor contributing to the differences in apoA-I levels between men and postmenopausal women. Moreover, plasma triglyceride levels are important determinants of HDL size and apoA-I catabolism. (Arterioscler Thromb Vasc Biol. 2000;20:801-806.)