Atherosclerosis 156 (2001) 363 – 372 Human apolipoprotein A-IV metabolism within triglyceride-rich lipoproteins and plasma Zhiyong Sun a , Alice H. Lichtenstein a , Gregory G. Dolnikowski b , Francine K. Welty a , Ernst J. Schaefer a, * a Lipid Metabolism Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts Uniersity, 711 Washington Street Boston, MA 02111, USA b Mass Spectrometry Laboratory, Jean Myer USDA Human Nutrition Research Center on Aging at Tufts Uniersity, Boston, MA 02111, USA Received 31 January 2000; received in revised form 13 June 2000; accepted 2 August 2000 Abstract In order to investigate the metabolism of apo A-IV within TRL and plasma, we assessed TRL and plasma apo A-IV kinetics in 19 and 4 subjects, respectively, consuming an average US diet for a 6-week period. At the end of this diet study, each subject received a primed-constant infusion of deuterated leucine over a 15 h time period with hourly feeding, and blood samples were drawn at 10 time points. TRL was separated by ultracentrifugation. Apo A-IV was isolated by immunoprecipitation and/or SDS-PAGE. Apo A-IV concentrations were determined by immunoelectrophoresis. Stable isotope tracer/tracee ratios were measured by gas chromatography/mass spectrometry, and the data were analyzed by multicompartmental modeling. The mean concentrations of plasma and TRL apo A-IV during the isotope infusion period were 21.0 3.2 and 0.66 0.25 mg/dl, respectively, and these values were 11.5 and 30.5% higher than those of fasting samples. The mean TRL and plasma apo A-IV residence times (RT) were 1.97 0.57 and 2.71 0.65 days, and transport rates (TR) were 0.17 0.19 and 3.90 1.24 mg/kg per day, respectively. There were significant correlations between TRL apo A-IV concentrations and TR (r 2 =0.79, P 0.001), and between TRL apo A-IV pool size and TRL cholesterol levels (r 2 =0.29, P =0.02). Our data indicated that; (1) TRL apo A-IV has a RT of 1.97 days which is similar to that earlier reported for HDL apo A-IV; (2) Apo A-IV recirculates between TRL and other slowly turning over pools; (3) the primary determinant of TRL apo A-IV levels is its TR; and (4) there is no correlation between TRL apo A-IV and apo B48 fractional catabolism in TRL. © 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Aplipoprotein A-IV; Apolipoprotein B48; Kinetics; Metabolism; Stable isotope; Tracer; Triglyceride-rich lipoproteins; Diet www.elsevier.com/locate/atherosclerosis 1. Introduction Apolipoproteins, the protein components of lipo- proteins, play a major role in lipid absorption, trans- port, and metabolism. Lipoprotein and apolipoprotein abnormalities have been shown to be associated with the risk of developing coronary heart disease. Human apo A-IV was first reported in 1978 by Weisgraber and colleagues [1]. It is a glycoprotein containing 376 amino acid residues with a molecular weight of 46 000 Da [1–6]. Human apo A-IV is synthesized primarily in the enterocytes of the small intestine and is secreted into the circulation through the lymphatic system. When apo A-IV enters the circulatory system, its association, distribution, function and metabolism may be influ- enced by other lipoprotein particles, apolipoproteins, enzymes and lipid transport reactions. Most investiga- tors have reported that apo A-IV plasma concentra- tions are between 13 and 19 mg/dl in fasting samples [2,3,5 – 11]. However, Zaiou [12] and Bisgaier [13] re- ported that mean apo A-IV concentrations in fasting plasma were 23.5 and 37.4 mg/dl, respectively. Plasma apo A-IV exists mainly in three fractions; TRL, HDL Abbreiations: apo, apolipoprotein; FCR fractional catabolic rate; HDL, high-density lipoproteins; LDL, low density lipoprotein; LFF, lipoprotein-free fraction; RT, residence time; SDS-PAGE, sodium dodecyl sulfate polyacrylamide gradient gel electrophoresis; TR, transport rate; TRL, triglyceride-rich lipoproteins; VLDL, very low- density lipoproteins. * Corresponding author. Tel.: +1-617-5663100; fax: +1-617- 5563103. E-mail address: eschaefer@hnrc.tufts.edu (E.J. Schaefer). 0021-9150/01/$ - see front matter © 2001 Elsevier Science Ireland Ltd. All rights reserved. PII:S0021-9150(00)00663-8