Abstract
In this thesis, chylomicron (CM) and CM-remnant (CM-R) metabolism in humans was studied by the application of unique markers which label these lipoprotein particles from the stage of production by the enterocyte until, removal by hepatic receptor mediated processes. Retinyl palmitate (RP) is a vitamin A ester, which labels the CM/CM-R by behaving like the cholesterol ester (CE) which is carried in the core of these particles. Development of a mono-specific antibody to apolipoprotein (apo) B-48 and application of an enzyme linked immunosorbant assay (ELISA) enabled quantification of this apolipoprotein which is specifically located on the surface of CM/CM-R. The postprandial lipaemic response for all parameters were determined by the area under the time response curve (AUC). Plasma was separated by flotation ultracentrifugation, overlayered with saline (d=1.006 g/ml), to separate the triacylglycerol-rich lipoprotein (TRL) and infranatant fractions. To examine the effects of habitual low intensity exercise on the postprandial CM response to meals of varying fat contents, triacylglycerol (TAG), apo B-48 and RP were measured in the TRL fraction. 14 young men were recruited and divided into two groups, active and inactive, depending on their habitual activity level. Active subjects were defined as those who undertook aerobic activity for at least 3 sessions a week and the inactive group took no regular exercise. In response to meals of 20 g, 40 g and 80 g fat content, the active subjects showed lower postprandial CM response as assessed by the TRL-apo B-48 AUC, by 35%, 58% and 66% (p < 0.05), respectively. The TRL-RP response to these meals was also lower by 67%, 54% and 49% (p < 0.01 in each case), in the active subjects. Plasma lipoprotein lipase (LPL) activity was 50% (p < 0.05) higher in the active group following the 80 g fat meal. The metabolism of CM/CM-R in the postprandial state was studied in 9 middle aged men with non-insulin dependent diabetes mellitus (NIDDM) and in 6 matched controls. The subjects were given a standardised meal containing 50 g fat, followed by a 12 hr postprandial study. NIDDM subjects, compared with the controls, showed exaggerated plasma postprandial responses of apo B-48, TAG and RP of 3696 vs 1796 ug.ml-1.min (p < 0.005), 2525 vs 1243 mmol.L-1.min (p < 0.05), and 3957 vs 2923 umol.L-1.min (p < 0.05), respectively. In the TRL fraction, apo B-48, TAG and RP AUCs were 1021 vs 473 ug.ml-1.min (p < 0.05), 1307 vs 527 mmol.L-1.min (p < 0.05), and 2707 vs 2158 umol.L-1.min (NS = not significant), respectively. There was no significant difference in plasma lipoprotein lipase (LPL) activity, between the groups. The effect of bezafibrate on postprandial lipoprotein metabolism was investigated in a 6-week placebo controlled, cross-over trial in the same 9 NIDDM subjects. Bezafibrate reduced the AUC for plasma apo B-48, TAG and RP by 45% (p , 0.005), 30% (p < 0.005) and 46% (p < 0.01). The corresponding reduction following bezafibrate therapy in TRL apo B-48, TAG and RP were 62% (p < 0.005), 54% ( p < 0.005) and 49% (p < 0.05), respectively. Plasma LPL activity was increased after bezafibrate therapy (p < 0.005). The effects of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors on postprandial lipoprotein metabolism, and in particular, CM-R metabolism, were investigated in 20 middle-aged subjects with primary hypercholesterolaemia. Subjects were given a high fat meal and the CM/CM-R markers, apo B-48 and RP, were measured in various fractions of plasma, obtained by cumulative flotation ultracentrifugation, using a linear salt gradient. The Sf 20-400 fraction of plasma, containing the majority of plasma CM-R was isolated. Reductions in TAG, CE, retinyl ester (RE) and apo B-48 AUC, in this fraction, following 4 weeks of HMG CoA reductase therapy, were 61% (p < 0.01), 20% (p < 0.01), 13 % (NS) and 23% (p < 0.01), respectively. Additionally, the smaller, potentially atherogenic CM-R, as measured by apo B-48 AUC in the Sf 60-100 and Sf 20-60 fractions of plasma were reduced by 15% (p < 0.05) and 22% (p < 0.05), respectively, following HMG CoA therapy. These studies have established that measurement of apo B-48 and RP are useful in the study of the metabolism of CM/CM-R in health and disease.