Abstract
Dietary-derived lipoproteins, chylomicrons (CM) and CM remnants (CMR), have been implicated in the progression of cardiovascular disease. Retinyl esters are currently the most widely used method for monitoring CM metabolism. The availability, however, of a specific antisera to apo B-48, the protein uniquely associated with dietary-derived lipoproteins, has allowed more extensive investigation of CM and CMR metabolism. The effect of habitual, moderate levels of exercise (3 to 4 exercise sessions a week) on the lipaemic response to meals of varying fat content was assessed in young male subjects. Apo B-48, triacylglycerol (TAG) and retinyl ester were used as markers for CM particles. Active subjects had a lower response than an inactive group in all parameters measured over time after the meals. Lipoprotein lipase (LPL) activity levels measured at the end of the postprandial period were higher in the active group. The area under the time-response curves (AUC) for apo B-48 in the inactive group increased with increasing fat content of the meals, whereas the AUC for apo B-48 was the same after each meal in the active group. Validation of a specific ELISA for apo B-48 was carried out. Cross-reactivity of the antisera with low levels of apo B-100, the protein present on endogenous lipoproteins, was ruled out. The assay was specific and sensitive for measuring apo B-48 concentrations in the CM-enriched fractions. The use of the assay in the current format for plasma samples could not be fully assessed due to difficulties with isolating a pure, concentrated sample of apo B-100 and problems with reactivity between the secondary antibody used in the assay and plasma proteins. The assay was useful for showing postprandial patterns of changes in apo B-48 levels in plasma. The effects of meal frequency on the lipaemic response to a high fat test meal challenge were assessed in an intervention study. A nibbling diet was found to cause differences between the response of various parameters after the meal (NEFA-AUC, LPL activity, infranatant-TAG AUC and time to peak) compared with the normal meal frequency. The size and density distribution of CMR in plasma were investigated. Apo B-48 was found in the IDL and LDL fractions in both the postabsorptive and postprandial states. A comparison between the retinyl ester and apo B-48 responses in the postprandial studies showed that the time to peak retinyl ester level was delayed compared to apo B-48 and TAG. The importance of apo B-48 for studying the metabolism of CM and CMR metabolism was demonstrated.