The hypothesis that plasma chylomicrons in persons who ingest a cholesterol-rich diet are atherogenic is evaluated. Evidence is presented that in humans, and experimental animals, chylomicron remnants as well as low-density lipoproteins are taken up by arterial cells. In persons who do not have familial hyperlipoproteinemia, atherogenesis may occur during the postprandial period. Research directions that may contribute to the evaluation of chylomicron remnants as a risk factor for atherogenesis are discussed. Lipoprotein studies after administration of a test meal containing fat and cholesterol are urgently needed.

Atherogenesis: a postprandial phenomenon.

High density lipoprotein metabolism.

Characterization of human high-density lipoproteins by gradient gel electrophoresis

The role of the human intestine has been explored as a site of synthesis of apoA-IV, a major apoprotein of human intestinal triglyceride-rich lipoproteins. Intestinal biopsies were performed on normal volunteers while fasting and after lipid ingestion. Indirect immunofluorescence demonstrated a marked increase in immunofluorescence for apoA-IV during lipid absorption consistent with an increased intracellular content. ApoA-IV comprised 10-13% of chylomicron apoprotein and 24-30% of intestinal very low density lipoprotein (VLDL) as assessed by densitometry of sodium dodecyl sulfate gels of lipoproteins from chylous urine (mesenteric lymphatic-urinary fistula) and thoracic duct lymph (postoperative fistula). After one subject with chyluria ingested 40 g of corn oil, triglyceride excretion in urine was accompanied by an increased excretion of apoA-IV. 11.5 g of triglyceride and 81 mg of apoA-IV were recovered in the urine. In chylous urine 56% of apoA-IV was in the triglyceride-rich lipoproteins (chylomicrons and intestinal VLDL) and 44% in the d > 1.006-g/ml fraction. Normal plasma apoA-IV was 15.7+/-0.9 mg/dl (n = 14) whereas four subjects with abetalipoproteinemia had reduced levels 1.2, 7.6, 9.6, and 8.3 mg/dl, respectively. Lipid feeding in normal volunteers resulted in a rise in plasma apoA-IV (16.1+/-0.7 mg/dl to 18.5+/-0.7 mg/dl, n = 5, P 1.21-g/ml fraction. In lipemic plasma, 10% of apoA-IV was associated with triglyceride-rich lipoproteins and 90% with the d > 1.21-g/ml fraction. Agarose column chromatography of fasting plasma confirmed that the bulk of plasma apoA-IV is free, unassociated with lipoproteins. These results demonstrate that apoA-IV is present in human intestinal epithelial cells and is secreted as a chylomicron and VLDL apoprotein. Within fasting plasma most of the apoA-IV is found free, unassociated with lipoproteins. After lipid ingestion apoA-IV is also found in plasma chylomicrons indicating that some apoA-IV remains associated with chylomicrons in plasma during chylomicron metabolism, although some may be transferred from the chylomicron surface.

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Human Apolipoprotein A-IV: INTESTINAL ORIGIN AND DISTRIBUTION IN PLASMA

Publisher Summary Plasma lipoproteins have lower hydrated densities relative to the other plasma proteins; therefore sequential flotation ultracentrifugation has been the principal method used for their isolation and classification. This chapter describes the sequential flotation ultracentrifugation method and some important features for the proper use of sequential flotation ultracentrifugation, such as it is important to calculate the effect of differences in path length or it is virtually impossible to reduce the level of contaminating proteins, such as albumin, below detection limits with a single spin. The ability to process simultaneously large volumes or many small samples is one of the principal advantages of sequential flotation ultracentrifugation. However, the technique suffers from certain limitations, such as during the course of long centrifugation, extensive lipid peroxidation can occur. This problem may be minimized by the inclusion of appropriate chelating agents and antioxidants. Full fractionation and purification by ultracentrifugal methods are both lengthy and costly. Also sequential flotation ultracentrifugation separates lipoproteins according to density only, and the investigator must be aware that particles differing in stage of metabolism, composition, charge, and size may be combined in any of the fractions obtained with this method.

Sequential flotation ultracentrifugation.

Lipoprotein apoprotein metabolism.

Transfer of human lymph chylomicron constituents to other lipoprotein density fractions during in vitro lipolysis.

Human apolipoprotein A-I isoprotein metabolism: proapoA-I conversion to mature apoA-I.

Apolipoprotein A-I isoforms in human lymph: effect of fat absorption.

The daily transport of human plasma apolipoproteins A-I and A-II, triglyceride, and total cholesterol from the thoracic duct lymph into plasma was measured in two subjects before and three subjects after renal transplantation. Lymph triglyceride transport was ~83% of the daily ingested fat loads, whereas lymph cholesterol transport was consistently greater than the amount of daily ingested cholesterol. Lymph apolipoprotein transport significantly (P < 0.05) exceeded the predicted apolipoprotein synthesis rate by an average of 659±578 mg/d for apolipoprotein A-I and 109±59 mg/d for apolipoprotein A-II among the five subjects. It is estimated that 22-77% (apolipoprotein A-I) and 28-82% (apolipoprotein A-II) of daily total body apolipoprotein synthesis takes place in the intestine.

Lymph high density lipoprotein particles are mostly high density lipoprotein2b and high density lipoprotein2a and have a greater overall relative triglyceride content and a smaller relative cholesteryl ester content when compared with homologous plasma high density lipoproteins. The major quantity of both lymph apolipoprotein A-I (81±8%) and apolipoprotein A-II (90±11%) was found within high density lipoproteins with almost all of the remainder found in chylomicrons and very low density lipoproteins.

The combined results are consistent with a major contribution of the intestine to total body synthesis of apolipoprotein A-I and apolipoprotein A-II. An important role of lymph in returning filtered apolipoprotein to plasma in association with high density lipoproteins is proposed. Accompanying the return of filtered apolipoprotein to the plasma is a probable transformation, both in size and composition, of at least some of the lymph high density lipoprotein2b and high density lipoprotein2a particles into high density lipoprotein3.

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E J Schaefer

Papers

Lipoprotein apoprotein metabolism.

Transfer of human lymph chylomicron constituents to other lipoprotein density fractions during in vitro lipolysis.

Human apolipoprotein A-I isoprotein metabolism: proapoA-I conversion to mature apoA-I.

Apolipoprotein A-I isoforms in human lymph: effect of fat absorption.

Transport of apolipoproteins A-I and A-II by human thoracic duct lymph.